Alliances played an important role in WWI because, there were two main alliances in Europe the Triple Alliance: Germany, Austria-Hungary, and Italy, and the rivals Triple Entente: the Soviet Union, Britain, and France. These alliances are what caused the war to rise rapidly from what should have only been regional conflict into a war throughout Europe.
The Alliance system was a key factor in the causation of WWI. Germany was scared because it was surrounded by two rival allied powers, France and Russia. Great Britain joined forces with Russia and France because it feared a Germany, the system of alliances between Europeans in the early 1900s helped in causation of WWI by causing a relatively small conflict, this was because, first, the alliance system gave the courage to some nations to act more aggressively and, second, that nations step up to defend their allies and this start the conflict which helped to cause WWI.
Alliances are possibly the best known cause of World War I. An alliance is a formal political, military or economic agreement between two or more nations. Military alliances usually contain promises that in the event of war or aggression, one signatory nation will support the others. It can also range from financial or logistic backing, like the supply of materials or weapons, to military mobilization and a declaration of war. Alliances may also contain economic elements, such as trade agreements, investment or loans. By 1914, the Great Powers of Europe had shuffled themselves into two alliance.