Final answer:
The United States entered World War I due to factors such as Germany's unrestricted submarine warfare, the impact of the Zimmermann Telegram, and economic interests linked to the Allied powers, despite initial efforts to maintain neutrality.
Step-by-step explanation:
The factors that led the United States to enter World War I were multifaceted and complex. Initially, the US under President Woodrow Wilson aimed to remain neutral, avoiding entanglement in international military conflicts.
However, as Europe plunged into war and commerce across the Atlantic was threatened, neutrality became unsustainable. The unrestricted submarine warfare practiced by Germany, particularly the sinking of the Lusitania which killed American citizens, was a pivotal factor.
Moreover, the Zimmermann Telegram, in which Germany proposed a military alliance with Mexico against the US, further galvanized public opinion in favor of war.
Domestically, while the US had a small volunteer army with limited impact on a global scale, the economic ties with the Allied powers were strong. American businesses and banks sided with the Allies, advocating for intervention.
Despite internal divisions — with many Irish immigrants opposing support for Britain and the significant German population potentially against aligning with the Allies — the collective push for safeguarding neutral rights and the freedom of the seas made entry into the war a firm stance for the US government.
The Selective Service Act of 1917, raising a large army, and efforts to sway public opinion in favor of the war, such as through propaganda, showcased the US's transition from neutrality to engagement in World War I.