Final answer:
The U.S. maintained neutrality in the first three years of World War I mainly due to isolationist policies and public opposition to entering the war. Neutrality Acts, and President Woodrow Wilson's pacifist stance, supported by a diverse immigrant population wary of splitting allegiances, played key roles. However, events like unrestricted submarine warfare and the Zimmermann Telegram ultimately led to U.S. entry into the war in 1917.
Step-by-step explanation:
The policies that kept the United States from entering World War I in its first three years were rooted in a deep-seated sentiment of isolationism and a desire to avoid entanglement in European conflicts. Key to this isolationist policy were the Neutrality Acts of 1935, 1936, and 1937, which prohibited Americans from selling arms, providing loans, or traveling on ships of nations at war. President Woodrow Wilson, who was re-elected in 1916 under the slogan “He kept us out of war,” was a pacifist determined to keep the U.S. neutral, influenced by the diverse immigrant population and the fear that taking sides could divide the nation.
Wilson’s efforts to avoid war were bolstered by public opinion against intervention, promoted by a variety of groups and influencers who saw the conflict as irrelevant to American interests. It was not until the strategic concerns of a potential German victory and the Bolshevik threat in Russia became pronounced in 1917, that some began to advocate for U.S. involvement.
The sinking of U.S. ships by German U-boats, notably the Lusitania in 1915, and the Zimmermann Telegram, eventually tested the U.S. stance on neutrality. Despite the initial resistance, the combination of strategic concerns and provocations eventually led President Wilson to ask Congress to declare war on Germany in April 1917, thus ending the United States' neutrality and joining the conflict.