- Isolationism and Non-Intervention: At the start of the war in 1914, the US held a strong tradition of isolationism, emphasizing neutrality and avoidance of foreign conflicts. Many Americans believed that the US should focus on domestic affairs, avoid European entanglements, and pursue a peaceful foreign policy.
- Lack of Direct Threats: Initially, the US did not perceive an immediate threat to its national interests or security from the events in Europe. While there was sympathy for the Allied powers (Britain, France, and Russia), the US was reluctant to intervene directly in a conflict that seemed geographically and politically distant.
- Trade with Both Sides: The US maintained significant economic ties with both the Allied powers and Germany. Trade, particularly with Britain and France, was essential for the US economy, while trade with Germany, though disrupted by the war, still had economic importance. The disruption of trade with both sides would have had negative economic consequences for the US.
- German U-Boat Attacks: The most significant factor that ultimately led to the US entering the war was the sinking of the British passenger ship Lusitania by a German U-boat in 1915, causing the loss of 128 American lives. This event, along with other incidents involving attacks on US merchant vessels, fueled public outrage and increased support for intervening on the side of the Allies.
- Wartime Diplomacy and the Zimmerman Telegram: Diplomatic tensions escalated as Germany pursued unrestricted submarine warfare, sinking merchant ships without warning. In early 1917, the British intercepted and released the now infamous Zimmerman Telegram, a secret proposal from Germany offering Mexico an alliance against the US. The telegram's disclosure further galvanized public opinion in the US, leading to a declaration of war on Germany on April 6, 1917.