This assassination led to Austria-Hungary declaring war on Serbia. When Russia began to fight because of it's alliance with Serbia, Germany declared war on Russia. Thus began the expansion of the war to include all those involved in the mutual defense alliances.
The sinking of the Lusitania
Countries throughout Europe made mutual defense agreements that would pull them into battle. These treaties meant that if one country was attacked, allied countries were bound to defend them.
German attempt in the Zimmermann Note to get Mexico
the invasion of neutral Belgium and stories of German atrocities in the country which shocked and outraged the Americans. Stories of unarmed civilians being killed and small towns being destroyed circulated throughout the press.
US Declaration of War
German U-boat sunk the British passenger ship Lusitania off the coast of Ireland. Over 1,000 passengers were killed, including 128 Americans.
Germany sent a letter suggesting that if the US should declare war on Germany, Mexico should declare war on the US In return, Mexico would get back the territory lost in the Mexican-American War
Entry into World War I, 1917. On April 2, 1917, President Woodrow Wilson went before a joint session of Congress to request a declaration of war against Germany.