Many things. The relationships between the European countries was getting more and more strained. The Germans had quite a large role in this, they sent a warship to (then French held) Mexico and also entered in an arms race with Britain over the new dreadnought class battleship. Another factor was the Balkans. These were a group of small independent countries in South Europe. Mainly Russia and Austria wanted those, but Germany also wanted control over them and possibly France as well. Russia tried to win popularity with the Slavic people who inhabited the Balkans by promising to protect them if Austria attacked. Also there was a system in Europe designed to stop war by fear. It was the alliance system. Austria, Germany and Italy were on one side and Great Britain, France and Russia were on the other. When a young Serb (Serbia being one of the Balkan countries) assassinated Archduke Franz Ferdinand (basically the prince of Austria) the Austria King gave Serbia an ultimatum. They failed to agree to the demands and Austria attacked. As Russia had promised it began to mobilise its armies to attack Austria. On seeing this, Austria's ally Germany put into effect a plan called the Schleifen plan. They expected Russia to be slow to mobilise her troops and during this time they planned to inflict a quick defeat on France through Belgium with the full might of their armies and then turn their attention to the (now mobilised) Russian army. In an attempt to protect their ally France, Great Britain warned Germany that if they moved their troops into Belgium to reach France then she would send over some troops to help out. Germany didn't listen and Britain joined the war. Now the alliance system, which had been designed to prevent war had brought war to the world. As, quite naturally, the competitors in the war began to bring in help from their empires. Britain had the help of the Indians, Canadians, Jamaicans, Australians, Russia hardly needed help as it already covered 1/6 of the worlds land mass. I fail to remember what aid the Germans and Austrians received, however, I do know that all have them had help from some areas of Africa. Now, pretty much the whole world was involved except the USA, who eventually joined the war in 1917 after much nagging from British politicians and also as a result of a German Unter-See-Booten, or you-Boat, sinking an American ship as it came to deliver supplies to Britain