Many of the European powers had entered into alliances requiring them to defend each other.
The systems of alliances and military plans which were put in place before World War I presupposed a major war between the countries which were tied together with alliances. Because the Triple Entente had Britain, France and Russia as allies, Germany thought if a war began it would need to fight on two fronts -- west and east. So German Field Marshall Alfred von Schlieffen drew up war plans that said attack France first, quickly, and then hold that territory while deploying forces to contend with Russia in the east.
So then here's how it happened. When an Austrian prince and his wife were assassinated in Serbia, the Austrian Empire threatened the nation of Serbia with retaliatory action (even though the assassination was carried out by a terrorist group, not the Serbian government). Russia responded to Austria's threat, because Russia was bound to protect its Slavic ally, Serbia. Germany responded to the mobilization of Russian troops, and when Germany declared war on Russia in 1914, they implemented the Schlieffen Plan and went on attack vs. France. That pulled France and Britain into the war immediately as well, and the war spread and became a global conflict.
Stay inside home at all times
The Battle of Lexington and Concord was the Start of the American Revolution ( April 19, 1775) (“Shot heard ‘round the world) It was one of the last straws if peace to be taken between the States and Britain (The Declaration of Independence was the last straw and was what cause the stamp of the war to be put down in 1776)
