Why would imperialism be considered the main cause of Worl War I?

Answer 1

World War I was the result of leaders' aggression towards other countries which was supported by the rising nationalism of the European nations. Economic and imperial competition and fear of war prompted military alliances and an arms race, which further escalated the tension contributing to the outbreak of war.
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Imperialism
Another factor which contributed to the increase in rivalry in Europe was imperialism. Great Britain, Germany and France needed foreign markets after the increase in manufacturing caused by the Industrial Revolution. These countries competed for economic expansion in Africa. Although Britain and France resolved their differences in Africa, several crises foreshadowing the war involved the clash of Germany against Britain and France in North Africa. In the Middle East, the crumbling Ottoman Empire was alluring to Austria-Hungary, the Balkans and Russia.
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Imperialism and World War I Timeline

1854 Ostend Manifesto issued by U.S. ministers to Europe. The ministers write up a document that urges the U.S. to annex Cuba for the security of slavery, and that if Spain refuses to sell the island, it should be taken by force.
1856 Congress acts to authorize the U.S. annexation of any small island that is unclaimed by other governments. In 1857, they will annex Jarvis Island and Bakerís Island, located in the mid-Pacific, and in 1858, Howardís Island.
1858 The first American consul to Japan completes an agreement between the two countries which opens up additional ports for trade, grants residence rights to American citizens, and establishes diplomatic relations.
1858 President Buchanan and Queen Victoria communicate greetings across the new Atlantic cable.
1858 President Buchanan urges Congress to give him the authority to purchase Cuba.
1859 Great Britain signs a treaty to give up its land in Central America.
1867 The U.S. acquires Alaska from Russia, thanks to deft negotiations by Secretary of State Seward.
1867 The U.S. annexes the Midway Islands.
1868 The Burlingame Treaty is issued, a treaty between the U.S. and China is concluded, establishing policies of commerce and friendship.
1871 The U.S. attempts to get a favorable treaty with strategic Korea by using force. Naval vessels bombard and destroy five Korean forts but retire empty-handed.
1871 The Treaty of Washington is formalized between the U.S. and Great Britain. It provides for a joint commission to settle fishing and boundary disputes.
1878 The U.S. acquires a naval base in Pago Pago, Samoa
1880 President Hayes warns that any canal across the isthmus of Panama will be strictly under U.S. control.
1881 Secretary of State James Blaine declares that the Hawaiian Islands are part of the American system and thus come under the intent of the Monroe Doctrine.
1887 The Senate ratifies a treaty with Hawaii guaranteeing the U.S. sole rights to build a naval base at Pearl Harbor.
1893 Queen Liliuokalani of Hawaii overthrown by John L. Stevens, U.S. ambassador to Hawaii, and powerful planters led by Sanford P. Dole.
1894 The Senate declares that Hawaii is to keep its own government, and warns that any nation interfering with that government will be considered hostile to the U.S.
1895 Cuban insurgents revolt against Spanish rule, supported by monies from American sugar planters. General Valeriano Weyler is sent from Spain to quell the revolt. Disgusted by Weylerís "reconcentration" policy, Americans sympathize with the Cubans. The "Yellow Press," led by William Randolph Hearst and Joseph Pulitzer, fans the flames of imperialism.
1898 The de Lôme letter is published in Hearstís New York Journal, characterizing President McKinley as feebleminded.
1898 The Battleship Maine is destroyed in Havana harbor, killing 260 of its crew.
1898 President McKinley delivers his War Message to Congress, a war resolution is adopted, and Spain breaks off diplomatic relations with the U.S.
1898 The "Rough Riders," or volunteer cavalry, is organized in April. Theodore Roosevelt will resign his post in the Navy to take up commission of lieutenant colonel in the Rough Riders.
1898 On May 1, Admiral Dewey attacks Spainís holdings in the Philippines by engaging the Spanish fleet in Manila Bay. The battle lasts seven hours. Spainís losses are 381 sailors and all its ships; there is no damage to U.S. vessels, and only eight men are wounded.
1898 On June 10, 647 American Marines land at Guantanamo Bay, beginning the invasion of Cuba. 17,000 American troops, including Theodore Rooseveltís "Rough Riders," invade Santiago on June 20.
1898 On July 1, Theodore Roosevelt leads the black Ninth and Tenth Regiments into a charge up San Juan Hill. By the end of the day, San Juan Hill is taken.
1898 The Hawaiian Annexation treaty is signed by McKinley.
1898 On July 26, three months after it began, the "splendid little war" is ended. The Treaty of Paris, ratified in December by a vote of 57-27, declares that Spain cede the Philippines, Guam, and Puerto Rico to the U.S.
1899 Secretary of State John Hay requests U.S. ambassadors to countries already having commerce, treaties, and long-term leases with China to ask for an "open door" policy by which all nations receive equal treatment from China.
1899 Emilio Aguinaldo leads an insurrection against American presence in the Philippines.
1900 The Foraker Act confirms Puerto Rico as an unconsolidated territory of the U.S. The president, with the advice and consent of Congress, appoints an executive council; the executive council appoints those under it.
1900 The fiercely nationalist Boxer Rebellion erupts in China.
1900 McKinley wins the Presidential election for a second term. Theodore Roosevelt is vice President.
1901 President McKinley is shot by Anarchist Leon Czolgosz. Forty-two-year-old Theodore Roosevelt takes the oath of office.
1901 Emilio Aguinaldo is captured by American forces, ending the Philippine-American war with American victory.
1901 The Supreme Court asserts, in what are known as the Insular Cases, that the Constitution does not apply immediately to annexed territories but that the privileges of Unites States citizenship must be conferred specifically by Congress. This verdict specifically targets the Philippines.
1901 The Hay-Pauncefote Treaty is signed between Great Britain and the U.S. It authorizes the U.S. to build, operate, and fortify a canal across the Central American Isthmus.
1901 The Platt Amendment is issued by Congress, which states that American troops will not withdraw from Cuba until the following conditions are included in their constitution: Cuba will not sign any agreement with a foreign power which will limit its independence; only the United States will be allowed to intervene to preserve Cuban independence and law and order; the Cubans agree to lease or sell naval stations to the United States.
1902 The Philippine Government Act is passed by Congress. It authorizes a commission, picked by the President with the advice and consent of Congress, to run the islands and declares the inhabitants to be citizens of the archipelago, not of the United States. On July 4, a civil government is established by Presidential order.
1903 The Hay-Herran Treaty is signed by the Colombian charge at Washington. Provisions include a 100 year lease on a 10-mile wide strip in the Panamanian province of Colombia. The price is $10,000,000 and an annual rental of $250,000. Colombia rejects the proposal.
1903 Instigated by a triumvirate of Panamanian businessmen, French agents of the Panama Canal Company and United States Army officers, Panama makes plans to secede from Colombia. In October, Roosevelt orders three ships of the United States Navy to steam toward the area. on November 2 they are ordered to prevent Colombia from landing troops in the Panamanian province. Two days later, Panamanian independence is declared. Secretary Hay recognizes the new Panamanian government in the quickest recognition then given to a foreign country by the United States.
1903 In November, the United States and Panama sign the Hay-Buneau-Varilla treaty giving the United States permanent rights to a 10-mile wide strip of land in return for $10,000,000 and an annual charge of $250,000 after nine years.
1904 The seven-man Panama Canal Commission is appointed by Roosevelt to complete the canal. With America becoming ever more intricately bound into the politics of the Atlantic and the Pacific, the project now has more urgency than ever.
1904 The Roosevelt Corollary is articulated for the first time in his annual message to Congress. The principle states that since America, by way of the Monroe Doctrine, has forbidden foreign interference in the Western Hemisphere, the United States has a responsibility to insist upon proper redress for wrongs inflicted upon a foreign state by any country within the U.S. sphere of influence. The Corollary is tested the following year when the United States begins supervision of the Dominican Republicís national and international debts, until foreign creditors have been recompensed.
1905 Roosevelt urges Russia and Japan, which have been at war since February 1904, to negotiate for peace. Inviting the two powers to Portsmouth, New Hampshire, the President uses a strong hand to help the belligerents reach resolution. Roosevelt will win the Nobel Peace prize for his mediation. Meanwhile, Secretary of War Taft is in Tokyo to negotiate the Taft-Katsura Agreement, which recognizes Japanís dominance over Korea in return for its promise not to invade the Philippines.
1907 Following the Roosevelt Corollary wherever it leads, the United States Marines are sent to Honduras to help quell a revolution there.
1908 William Howard Taft is elected President. James S. Sherman is vice president. Taft articulates slight variation on Rooseveltís "big stick" that comes to be known as "Dollar Diplomacy." Taft says that he wishes to substitute "dollars for bullets" in his dealings with Latin America, with American capital bringing healthy reform to corrupt governments.
1908 The Root-Takahira agreement is reached, by which Japan and the United States promise to maintain the status quo in the Pacific.
1912 Woodrow Wilson is elected President. He is the first Democratic President since Grover Cleveland. Wilson is a progressive reformer, with a solid Democratic House, about to set his reforms.
1913 The Panama Canal is open for shipping.
1914 In June, Archduke Francis Ferdinand, Crown Price of Austria, is murdered by Gabriel Princips and some Pan-Slav fanatics in Sarajevo. The Austro-Hungarian Government sends an ultimatum to Serbia and five days later declares war. By mid-August, Germany has declared war on Russia and France; Great Britain declares war on Germany; Austria-Hungary declares war on Russia.
1914 The United States issues a formal statement that it will remain neutral in the European wars. It proposes that the belligerents stand by international law which states that neutral seas are open seas, and that neutral shipping is thus protected.
1915 Wilson warns Germany that the United States will hold it "to a strict accountability" for "property endangered or lives lost." German submarine warfare is taking a serious toll on neutral shipping, including American. Germany declares water around British Isles a war zone.
1915 The great ocean-going British passenger ship Lusitania is sunk without warning in May, losing 1198 of its 1924 passengers. There are 114 Americans aboard, and the tragedy strikes home. According to the Germans, the ship is carrying munitions, though the British deny this.
1915 Secretary of State Bryan sends the first Lusitania note to Germany, demanding disavowal of the attack of the Lusitania and immediate reparations. The Germans do neither. Bryan resigns on the grounds that as a pacifist he cannot sign the strong second Lusitania note which has been written by Wilson and other members of his cabinet.
1915 Wilson sends his second Lusitania note. Germany must make reparations and "prevent the recurrence of anything so obviously subversive of the principles of warfare." He goes on to say, "The lives of non-combatants cannot lawfully or rightfully be put in jeopardy by the capture or destruction of an unresisting merchantman." Germany claims it will allow Americans to travel in well-marked, neutral, unarmed ships. Wilson sends a third note, warning that future infringement of American rights will be deemed "deliberately unfriendly."
1915 In August, a military training camp is set up to begin the training of civilians under the initiative of General Leonard Wood, Grenville Clark, and Theodore Roosevelt. Wilson will not endorse the idea of "preparedness" until November.
1915 United States bankers arrange a much needed $500,000,000 loan to the British and French.
1915 In December, Wilson asks for a standing army of 142,000 and a reserve of 400,000. By March of 1916, the Senate has unanimously resolved to bring the army to full strength. By the end of June, the House will pass a $182,000,000 army appropriations bull, the largest military budget so far.
1916 In March, German U-boats sink the French vessel Sussex which is steaming through the English channel. The ship is unarmed and three Americans lose their lives. Secretary of State Lansing warns Germany that the United States will break off diplomatic relations unless these attacks are discontinued. In the Sussex pledge, Germany asks the U.S. to compel the allies to end their trade blockade. Wilson turns down the plea. By now the general public sympathy is with Wilson and his cabinet.
1916 Wilson wins a second term, having campaigned on the slogan, "He kept us out of war."
1916 Wilson appears before Congress and outlines his conditions for American participation and asks that the present conflict be resolved in such a way a to leave no aftermath of bitterness in the hearts of the defeated, a "Peace without victory." This is a concept unappealing to both warring factions.
1916 Lewis Terman improves upon Alfred Binetís intelligence tests, and formalizes the "IQ" test. These tests will be used by the War Department in 1918 to test American military recruits.
1917 Germany declares that it will resume unrestricted submarine warfare. Neutral ships, armed or unarmed, sailing into a German war zone, will be attacked without warning. In response, Wilson breaks off diplomatic relations with Germany. The same day, the American steamship Housatonic is sunk without warning.
1917 British secret service agents intercept a telegram for the German foreign minister Zimmerman to the German ambassador to Mexico. They now turn the decoded telegram to Wilson: Zimmerman, foreseeing war with the United States as inevitable, has instructed the German ambassador to instigate Mexican entrance into the war on the side of the Central Powers in return for New Mexico, Texas, and Arizona. This pushes Wilson to the brink of war with Germany. The telegram is released to the public with responds with predictable anger.
1917 The Jones Act makes Puerto Rico part of United States territory, making its inhabitants U.S. citizens.
1917 In April, Wilson asks Congress to declare war on Germany. "The world," he says in a famous phrase, "must be made safe for democracy." The vote in Congress is overwhelmingly in favor of war.
1917 Wilson forms the Committee on Public Information (CPI) headed by George Creel. Creel hires progressives and recruits thousands of people in the arts, advertising, and film industries to publicize the war. He works out a system of voluntary censorship in the press, and enlists 75,000 "four-minute men" to give public speeches about the "bloodthirstiness" of Germans.
1917 In May, the Selective Service Act authorizing registration and draft of all men between 21 and 30 military service is passed by Congress.
1917 Herbert Hoover is put in charge of the Food Administration, designed to increase food production and distribution.
1917 The War Industries Board is created, headed by Bernard Baruch, overseeing the production of all American factories. It launches the streamlining of production, allocates raw materials, sets prices and output.
1917 The Fuel Administration, headed by Harry Garfield, introduces daylight savings time, rations coal and oil, and imposes gasless days. The Railroad administration and the War Shipping Board coordinate arms shipment.
1917 In August, Frank Little, an anti-war labor leader, is tortured and hanged by vigilantes.
1917 The Espionage Act is passed by Congress, which imposes sentences of up to twenty years in prison for anyone found guilty of aiding the enemy, obstructing the recruitment of soldiers, or encouraging disloyalty. It allows the postmaster general to remove from the mails any materials that may incite treason or insurrection. In 1918, Congress passes the Sedition Act, imposing harsh penalties for anyone using "disloyal, profane, scurrilous, or abusive language" about the government, flag, or armed forces.
1917 In November, the Bolsheviks overthrow the Kerenski government in Russia and will soon make peace with Germany. The United States does not immediately recognize the new government.
1917 The United States declares war against Austria-Hungary in December.
1918 In January, Wilson sets forth his famous "Fourteen Points" for peace in the world. Most of these are specific to German borders, Polish, Turkish, and Belgian sovereignty, freedom of the seas, and the like, but the final point will cause ripples in the international world for many years. It asks for a "general association of nations... under specific covenants for the purpose of affording mutual guarantees of political independence and territorial integrity to great and small states alike."
1918 In April, German-born Robert Prager is lynched in Missouri by a mob. The mobís members are acquitted.
1918 In May, Wilson names Felix Frankfurter the head of a new War Labor Board. It standardizes wages and hours, protects the right of labor to bargain collectively, and uses tactics to discourage strikes.
1918 Racial tensions increase largely as a result of the movement of southern blacks to the North in order to fill wartime jobs, often encouraged by northern labor agents. Race riots take place in St. Louis, Washington, D.C., Chicago, New York City, and Omaha, killing a total of hundreds of blacks and many whites.
1918 In October, Germany forms a parliamentary government, and with the collapse at the front, mutiny in the navy and revolution in Munich, the Kaiser abdicates. An armistice is signed in November.
1919 In January, the Versailles peace conference begins.
1919 The 18th Amendment to the Constitution is ratified.
1919 In Schenk v. United States, the Supreme Court finds that the Espionage Act does not violate the First Amendment. Holmes agrees with the majority that there exists a standard of "clear and present danger" which may restrain free speech. Under this ruling Eugene V. Debs is sentences to ten years in prison for interfering with the draft.
1919 The peace conference ends in June with the signing of the Versailles Treaty. Germany is asked to admit her guilt, to give up the rich Alsace-Lorraine and her overseas colonies, and pay reparations of some $15,000,000,000. Further trouble will result from clauses in the treaty that prevent German rearmament, and which provide for an indemnity of an indeterminate amount designed to keep the defeated nation perpetually poor. The final treaty does not follow closely Wilsonís Fourteen Points. Ultimately, the issue dearest to Wilsonís heart, the League of nations, is accepted by all signatories. However, because of the League issue, the Senate will
never ratify the treaty.
1919 Instead of negotiating the Versailles Treaty and the League of Nations Covenant with the Senate, Wilson goes on a tour of the country to rouse public opinion in favor of the project. The President is already quite sick and proceeds against the advice of his doctors. He suffers a stroke later that month. He never fully recovers from the effects. Meantime, the Senate refuses to ratify the Treaty of Versailles, 55-39.

Answer 2

Well after these pigs decided to imperialize they got greedy. They wanted more. Hence the War. What do you think continued into the second war. The fascists were conquering (imperializing) Europe. Europeans had already concurred 98% of Africa, south america, the Caribbean, Asia they just turned on each other. CONQUER is the name of their game. Hope i helped.