Final answer:
World War 1 was caused by tensions between the Great Powers and the US entered the war due to Germany's unrestricted submarine warfare and other factors.
Step-by-step explanation:
Causes of World War 1:
The underlying causes of World War 1 were nationalism, opposition to foreign rule, and simmering rivalries between the Great Powers that were exacerbated by treaties requiring allies to enter a war once it began. These tensions eventually led to the assassination of Archduke Franz Ferdinand of Austria-Hungary by a Serbian nationalist in 1914, which triggered a chain reaction of declarations of war between the involved nations.
US entry into the war:
Initially, the United States tried to maintain its position of neutrality in World War 1. However, as the war engulfed Europe and the belligerents' total war strategies targeted commerce and travel across the Atlantic, it became clear that the US could not stay out of the conflict. The final tipping point came when Germany began unrestricted submarine warfare, sinking American ships. This, combined with other factors like economic ties with the Allies and Wilson's commitment to the freedom of the seas, led the US to enter the war in 1917.