when did the U.S finally enter the war against Hitler?

Answer 1

Shortly after the Japanese sneak attack on Pearl Harbor, Hitler declared war on the USA.

"Hitler Declares War on the United States

(December 11, 1941)

Deputies, Men of the German Reichstag! A year of events of historical significance is drawing to an end. A year of the greatest decisions lies ahead. In these serious times, I speak to you, Deputies of the German Reichstag, as to the representatives of the German nation. Beyond and above that, the whole German people should take note of this glance into the past, as well as of the coming decisions the present and future impose upon us.
After the renewed refusal of my peace offer in January 1940 by the then British Prime Minister and the clique which supported or else dominated him, it became clear that this war-against all reasons of common, sense and necessity-must be fought to its end. You know me, my old Party companions: you know I have always been an enemy of half measures or weak decisions. If the Providence has so willed that the German people cannot be spared this fight, then I can only be grateful that it entrusted me with the leadership in this historic struggle which, for the next 500 or 1,000 years, will be described as decisive, not only for the history of Germany, but for the whole of Europe and indeed the whole world. The German people and their soldiers are working and fighting today, not only for the present, but for the coming, nay the most distant, generations. A historical revision on a unique scale has been imposed on us by the Creator."

Part of the reason was that, since the USA had declared war on Japan, Hitler was "bound" by the Tripartite Pact, the Three-Power Pact Between Germany, Italy, and Japan, Signed at Berlin, September 27, 1940.

especially article 3:

"Germany, Italy and Japan agree to co-operate in their efforts on aforesaid lines. They further undertake to assist one another with all political, economic and military means when one of the three contracting powers is attacked by a power at present not involved in the European war or in the Chinese-Japanese conflict."

But another reason was Hitler's comtempt for the USA, his belief that it was a "degenerate society" and that it would have little or no influence in the outcome of the war.

Boy, oh, boy - was he ever wrong.

Answer 2

Hitler declared war on the USA on December 11 1941.
This prevented many american companies such as Standard Oil from giving any further support to the Nazis.
They are notorious for giving the Nazis the process for making synthetic rubber ( a strategic war material). This probably prolonged the war by several months.

Answer 3

The US had already been in an undeclared naval war since 1941. Following the incident with the Destroyer USS Greer President Franklin Roosevelt declared that the US Navy would attack on sight or detection any German U-boat.
After Pearl harbor the Germans decalred war on the USA on December 11, 1941.

Answer 4

actually declaring war is not entering until you have feet on the ground.
Germany and Italy declared war on the United States, which brought the US into the European Theatre of Operations.

The US 8th Army Airforce arrived in the United Kingdom during the late Spring/early Summer of 1942. The US Army came later.

Answer 5

December 11th, 1941 But the U.S. had entered the war unofficially many months earlier. U.S. naval forces were protecting convoys to England long before Pearl Harbor. There was actual combat between German submarines and U.S. ships. And Lend-Lease was supplying equipment and supplies to England for some time. So you take you pick. But it was NOT too late, perhaps just in time better fits the answer.

Answer 6

Officially speaking, it was December 1941, after the Pearl Harbor attack. You may remember that Japan, Italy, and Germany were allies in the Rome-Berlin-Tokyo Axis, so a declaration of war against one of them was a declaration of war against all.

However, there was an unofficial war for a number of years prior to that.

Answer 7

against the Japanese - after the attack on Pearl Harbour, Dec 1941.

Against the germans - a little later, say the begining of 1942

Answer 8

A little too late.