As a historical expert, I can provide an in-depth analysis of why the United States joined World War I. The decision was multifaceted, with several key factors influencing the U.S.'s entry into the conflict.
First and foremost, the **resumption of unrestricted submarine warfare** by Germany was a significant factor. This policy, which allowed German U-boats to sink any ship, including neutral ones, without warning, was a direct threat to American lives and economic interests. The sinking of the British passenger liner Lusitania in 1915, which resulted in the deaths of 128 Americans, was a shocking event that began to turn public opinion in the U.S. against Germany.
Secondly, the Zimmerman telegram played a crucial role. This was a coded message from the German Foreign Minister Arthur Zimmerman to the German ambassador in Mexico, proposing a military alliance with Mexico in the event of the U.S. entering the war against Germany. The message suggested that Mexico could reclaim lost territories in the American Southwest. The British intelligence intercepted this telegram, and its revelation to the U.S. public in 1917 was a major catalyst for anti-German sentiment and support for war.
Economic considerations were also important. The U.S. had significant economic ties with the Allies, particularly with Great Britain and France. American banks had loaned large sums to the Allies, and there was a concern that if the Allies were to lose the war, these loans would not be repaid. This economic self-interest was a driving factor in the U.S.'s decision to support the Allies.
Furthermore, there was a strong ideological component to the U.S.'s entry into the war. President Woodrow Wilson, who had been re-elected in 1916 on the slogan "He kept us out of war," eventually shifted his stance. He framed the war as a fight for democracy against autocracy, which resonated with the American public and provided a moral justification for entering the conflict.
Lastly, domestic politics and pressure from various interest groups within the U.S. also influenced the decision. There was a significant portion of the American population, particularly those of Irish, German, and other immigrant descent, who were opposed to entering the war. However, others, including many in the business community and those who saw the war as a way to project American power and influence, were in favor of intervention.
In summary, the U.S. entered World War I due to a combination of factors, including the threat to American lives from German submarines, the Zimmerman telegram, economic interests, ideological motivations, and domestic political pressures. These factors converged to create a situation where intervention was seen as necessary by the U.S. government and a significant portion of the American public.

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The United States entered the war because of the Germans' decision to resume the policy of unrestricted submarine warfare, and the so-called "Zimmerman telegram," intercepted by the British, in which Germany floated the idea of an alliance with Mexico.read more >>