What caused the ultimate downfall of the Papal States?

Question

Name one important thing that happened because of it.

Answer 1

Pius IX refused to accept the new political situation or to recognize the loss of papal temporal power, and he and his successors remained self-described “prisoners in the Vatican.” The question of the pope's relation to the Italian state was unsettled until the Lateran Treaty of 1929 set up the independent ecclesiastical state of Vatican City.
http://www.britannica.com/eb/article-9058317/Papal-States
Dissolution and Resolution
After the Counter Reformation (16th cent.) the spiritual power of the papacy grew while its political power waned. Papal troops, mostly Swiss and other mercenaries, offered almost no resistance to the French invaders under Napoleon Bonaparte (see Napoleon I) in 1796. Pius VI and his successor, Pius VII, saw their states curtailed, occupied, and twice abolished between 1796 and 1814. The Congress of Vienna fully restored (1815) the states of the papacy and placed them under Austrian protection. 6
Conspiracies and revolutions (notably of 1831 and 1848–49) characterized the following decades. Pius IX was liberal at his accession and granted his states a constitution, but the events of 1848 turned him against the revolutionists. During the Risorgimento, only French intervention at Rome prevented the total absorption of the Papal States. After the Austrians left (1859) Bologna and the Romagna, both united (1860) with the kingdom of Sardinia, as did Marche and Umbria. Giuseppe Garibaldi invaded the remaining Papal States twice but was prevented from taking Rome—in 1862 by the intervention of Victor Emmanuel II of Italy, and in 1867 by Napoleon III. 7
The fall of Napoleon permitted Victor Emmanuel to seize Rome in 1870. However, Pius IX refused to recognize the loss of temporal power and became a “prisoner” in the Vatican; his successors followed his example. The so-called Roman Question was only resolved in 1929 by the Lateran Treaty, which, among other things, established the Vatican City (see under Vatican).
http://bartelby.com/65/pa/PapalSta.html
The fall of Napoleon permitted Victor Emmanuel to seize Rome in 1870. However, Pius IX refused to recognize the loss of temporal power and became a “prisoner” in the Vatican; his successors followed his example. The so-called Roman Question was only resolved in 1929 by the Lateran Treaty, which, among other things, established Vatican City.
http://www.infoplease.com/ce6/history/A0860228.html