World War I was a direct result of a tangled system of secret alliances. Beginning after the unification of Germany in 1871, many European nations began secretly allying themselves with each other. Starting with the Dual Alliance between Austria-Hungry and Germany in 1879, the nations of Europe allied themselves in mutual protection pacts in such a way that if any one nation became the target of aggression, all of Europe would be pulled into war. That act of aggression occurred in Bosnia in 1914 when a young serb, Gavrilo Princip, assassinated Archduke Franz Ferdinand, heir to the Austro-Hungarian throne. Austria-Hungry declared war on the nation of Serbia soon after. Within a year, Austria-Hungry, Germany and Italy had allied themselves against France, Britain, Russia and the Ottoman Empire.
The after-effects of the war are numerous. Four empire disappeared; Austria-Hungry, the Ottomans, Germany and Russia. Four age-old royal lines were shattered; the Hapsburgs, Romanov’s, Hohenzollerens and the Ottman turks. Of the 60 million soldiers mobilized during the war, 8 million were dead, 7 million were disabled in some way and 15 million were seriously injured. 15% of German’s men aged 18-40 were gone, as were 17% of Austria-Hungry. A global famine also came after the war, killing 100,000 people in Lebannon and 10 million in Russia! One of the most important effects was that Germany was saddled with a huge war debt that bankrupted the nation and provided fertile ground for the rise of Hitler and the Nazi's, so one could argue World War I caused World War II
There were some positive effects as well. There were vast improvements in the way armies cared for wounded soldiers and veterens. Mental health care for soliders with PTS syndrome was beginning to be introduced and the League of Nations was formed. While the League itself proved to be ineffective, it was the precursor to the United Nations, a much more effective international organization.
...unfriendly state and in order to weaken Serbia, the Austrian government threatened war in the autumn of 1912 if Serbs were to acquire a port from the Turks. Austria appealed for German support, only to be rebuffed at first. In November 1912 Russia, humiliated by its inability to support Serbia during the Bosnian crisis of 1908 or the First Balkan War, announced a major reconstruction of its military. On November 28, in partial reaction to the Russian move, German Foreign Secretary Gottlieb von Jagow told the Reichstag, the German parliament, that “If Austria is forced, for whatever reason, to fight for its position as a Great Power, then we must stand by her”. As a result, British Foreign Secretary Sir Edward Grey responded by warning Prince Karl Lichnowsky, the German Ambassador in London, that if Germany offered Austria a “blank cheque” for war in the Balkans, then “the consequences of such a policy would be incalculable”. To reinforce this point, R. B. Haldane, the Germanophile Lord Chancellor, met with Prince Lichnowsky to offer an explicit warning that if Germany were to upset the balance of power in Europe by trying to destroy either France or Russia as powers, Britain would have no other choice but to fight the Reich. With the recently announced Russian military reconstruction and certain British communications, the possibility of war was a prime topic at the German Imperial War...