which was one major reason the United States entered World War 1 on the Allied side

One major reason the United States entered World War I on the Allied side was due to unrestricted submarine warfare by Germany. In an attempt to cut off Britain from her naval supplies, Germany began sinking ships without warning in the waters around Britain and Ireland. The most notorious incident was the sinking of the British passenger liner RMS Lusitania in 1915, which resulted in the deaths of 1,198 civilians, including 128 Americans. This act of aggression against non-combatant ships, including those belonging to neutral countries like the United States, outraged the American public and increased anti-German sentiment in the US.

The tipping point came with the interception and publication of the Zimmermann Telegram in 1917, in which Germany proposed a military alliance with Mexico against the United States, promising Mexico the return of territories lost during the Mexican-American War. The combination of these events, along with a desire to defend freedom of the seas and maintain trade routes, led President Woodrow Wilson to ask Congress to declare war against Germany, which they did on April 6, 1917.