The three contributing events to the start of World War I are:
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The assassination of Archduke Franz Ferdinand - This event is widely regarded as the immediate catalyst for the war, as the assassination of the Austro-Hungarian heir by a Serbian nationalist led to a series of political maneuvers and alliances being activated.
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The militarization of especially Germany and Great Britain - The arms race and militarization in Europe created an environment of tension and competition among the great powers, ultimately contributing to the outbreak of war.
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The sinking of the RMS Lusitania - This event played a significant role in turning public opinion in the United States against Germany, contributing to America's eventual entry into the war.
These events collectively illustrate the complex interplay of political alliances, nationalistic tensions, and public sentiment that led to the outbreak of World War I and America's involvement in it.