Francis Ferdinand (Archduke of Austria) and his wife Sophie (Duchess of Hohenberg) were assassinated during a visit to Sarajevo. Austria-Hungary blamed the Serbian for the assassination, asking for help from Germany. Serbia, however, were allied with Russia, Great Britain, and Italy. This led to Austria-Hungary declaring war on Serbia, beginning WW1.
Germans planned to execute the Shlieffen Plan, a plan devised by Field Marshal Aflred von Schlieffen for a one-front war against France. Russia deployed their troops much quicker than anticipated, though. The Russians ended up invading Germany, leading the German army to have to split their troops to up hold the Russian invasion.
U.S. Enters WW1
Attack the United States? -Germany
In the First Battle of Marne, the French and British army fought against the Germans that invaded Belgium and France. The battle was seen as one of the first victories of the Allied Powers of WW1. This saved Paris from the Germans and overthrew their plan of having a victory over France.
Treaty of Brest-Litovsk
Without warning, a British ocean liner called the Lusitania was torpedoed by a German u-boat without any warning beforehand. This led to more than 1,000 people being killed, including over 100 Americans on board. The sinking caused outrage from Britain and America, while many Americans wanted to remain neutral in the war.
The United States of America jointed World War 1 to help support the Allied Powers. Certain events contributed to this, including the Lusitania sinking and the Zimmerman Note, where Germans attempted to get Mexico to attack the U.S.. America provided many resources, providing a huge advantage for the Allied Powers against the Central Powers.
The Treaty of Brest-Litovsk was a peace treaty signed by the new Bolshevik government of Soviet Russia and the Central Powers. The signing of this peace treaty led to the ending of Russia's participation in World War 1. Because of the treaty, Germans were able to devote their resources to an offensive on the Western Front.