Final answer:
The United States entered World War I.
Step-by-step explanation:
The event that best completes this timeline is The United States entered World War I. The sinking of the Lusitania by a German U-boat in 1915 was one of the factors that led to the United States' entry into the war in 1917. The Zimmermann Telegram, intercepted in 1917, revealed a secret German proposal to Mexico to join the war against the United States, further influencing the decision to enter the war.
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