Final answer:
The U.S. entered World War I primarily due to unrestricted submarine warfare, which violated neutral rights, the Zimmermann Telegram posing a national security threat, and strong economic ties and ideological alignment with the Allies.
Step-by-step explanation:
The United States entered World War I due to several factors that shaped the country's decision to join the conflict. One of the primary justifications was the unrestricted submarine warfare policy adopted by Germany, which led to the sinking of passenger ships like the Lusitania and the Sussex, with Americans among the casualties. This act violated the traditional rights of neutral countries and threatened American sovereignty.
Another significant factor was the Zimmermann Telegram, in which Germany proposed a military alliance with Mexico against the United States, promising the return of territories such as Arizona, New Mexico, and Texas to Mexico upon victory. This posed a direct threat to the national security of the United States. Moreover, economic ties favored the Allies due to extensive trade relations and loans.
American banks and businesses had a vested interest in an Allied victory to protect their investments. Additionally, the ideological alignment with the democratic governments of the Allies against the autocratic Central Powers also influenced the decision. Prior to the declaration of war, President Woodrow Wilson emphasized neutrality and domestic policy, but his views evolved due to the repeated violations of neutral rights and the potential global impact of the conflict.
Despite the opposition from groups with loyalties to other nations within the US and the lack of a sizable standing army, the threat to national security, economic interests, and the principles of democracy and freedom ultimately led to the US Congress declaring war on April 6, 1917.