The United States entered World War I in April 1917, primarily in response to a combination of German actions and broader strategic interests, rather than a single event alone. Key factors included:
- Unrestricted submarine warfare: Germany resumed sinking ships bound for Britain, including American merchant vessels, in an effort to cut off supplies to the Allies. This threatened American lives and trade and shifted public opinion toward intervention [web results show discussions of submarine warfare as a catalyst; see historical summaries and primary statements by U.S. policymakers].
- The Zimmermann Telegram: a diplomatic communication from Germany to Mexico proposed a military alliance against the United States if war with Germany occurred, which outrageously inflamed American sentiment and helped push policy-makers toward war [web results reference the Zimmerman telegram as a contributing factor].
- Economic and strategic stakes: the United States had strong financial and trade ties with the Allies, including large loans and abundant exports of war supplies, creating both economic incentives and a desire to ensure favorable outcomes for global stability and commercial interests [web results describe U.S. loans and economic links with the Allies].
- Ideological and political considerations: public opinion gradually shifted from neutrality to supporting a war to defend democracy and to make the world “safe for democracy,” a framing popularized by President Woodrow Wilson in his war message to Congress in 1917 [web results indicate Wilson’s debated stance and the eventual shift in public mood].
Direct answer: The United States entered World War I in 1917 largely because Germany’s unrestricted submarine warfare threatened American lives and commerce, and because diplomatic and strategic factors—such as the Zimmermann Telegram and the desire to safeguard economic and political interests—made intervention both morally and pragmatically justifiable to U.S. leaders. This culminated in Congress’s declaration of war on Germany on April 6, 1917 [web results summarize these strands and timelines].
