The first immediate cause which made Austria- Hungary declare war on Serbia, was the assassination of the Archduke Francis Ferdinand at Sarajevo, by a Serbnationalist. But this wasn´t the main cause for the beginning of the war, other matters were involved. The spirit of intense nationalism after the French Revolution and the Napoleonic wars spread through mostEurope and political and economic rivalry among the nations began to grow since Britain´s imperial and economic power weakened.
Europe divided into two main alliances, the Triple Alliance ofGermany, Austria- Hungary and Italy and the Triple Entente of Britain, France and Russia. The Europe nations armed themselves for purposes of defense, but also in order not to find themselves alone ifwar did break out.
As Austria- Hungary declared war on Serbia; and Russia had promised Serbia to defend it, it mobilize against Austria. Germany warned Russia that continued mobilizationwould entail war with Germany and it did; the French began to mobilize on the same day and the next day, Germany declared war on France. German government informed the government of Belgium of itsintention to march on France through Belgium. The Belgian government refused to permit the passage of German troops and a Treaty guaranteeing the neutrality of Belgium in case of a conflict was signed byBritain, France and Germany. Germany didn´t respect the treaty so Britain declared war on it.
Germany nearly defeated Britain and France
United States President Woodrow Wilsondeclared a U.S. policy of absolute neutrality on the same day Britain declared war, 4 August, an official stance that would last until 1917 when Germany's policy of unrestricted submarine warfare - whichseriously threatened America's commercial shipping (which was in any event almost entirely directed towards the Allies led by Britain and France) - forced the U.S. to finally enter the war on 6 April...