1: The Zimmerman Telegram, in which Germany offered Mexico "substantial aid" in regaining Mexican territory lost to USA in the Mexican-American War (1846-48) if they joined the war on Germany's side and attacked USA.
2: Unrestricted submarine warfare by the Germans.This was sinking neutral US ships and killing US merchant seamen.Further, it was preventing legitimate imports and exports to and from the USA, thus harming the economy.
