Zimmerman Telegram: In January 1917, the British intercepted and decrypted a telegram sent by German Foreign Secretary Arthur Zimmerman to the German ambassador in Mexico. The telegram proposed an alliance between Germany and Mexico against the US, promising to help Mexico "reconquer" its lost territories in the Southwest if the US were to enter the war against Germany. This revelation fueled anti-German sentiments and influenced American public opinion in favor of intervening in the war.
Economic Interests: The Allies owed large debts to the United States for supplies and loans provided during the war. Siding with the Allies could ensure the repayment of these debts and maintain America's economic interests.
Public Pressure: There was growing public support in the US for aiding the Allied powers, driven by factors such as the sinking of the Lusitania, sympathy for Belgium's suffering under German occupation, and moral outrage over German atrocities.
Moral and Humanitarian Concerns: Many Americans were appalled by the atrocities committed by Germany and its allies, including the use of poison gas, deliberate destruction of civilian property, and mistreatment of prisoners of war.
Alliance with Great Britain: The US and Great Britain had deep historical ties, shared language and culture, and cooperated in various areas. These factors contributed to a natural inclination towards supporting Britain in the war.
Russian Revolution: The overthrow of the Russian monarchy in February 1917 and the establishment of the provisional government further weakened the Central powers allies and made it more likely that the Allies could win the war.