War between the Central European Powers (Germany, Austria-Hungary, and allies) on one side and the Triple Entente (Britain and the British Empire, France, and Russia) and their allies, including the USA (which entered in 1917), on the other side.
From The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide Battle zone in World War I between Germany and its enemies France and Britain, extending as lines of trenches from Nieuport on the Belgian coast through Ypres, Arras, Albert, Soissons, and Rheims to Verdun, constructed by both Germany and the Allies.
Agreement between two or more states to come to each other's assistance in the event of war. Alliances were criticized after World War I as having contributed to the outbreak of war, but the North Atlantic Treaty Organization (NATO) has been a major part of the post-1945 structure of international relations (as was the Warsaw Pact until its dissolution 1991).
International organization formed after World War I to solve international disputes by arbitration. Established in Geneva, Switzerland, in 1920, the League included representatives from states throughout the world, but was severely weakened by the US decision not to become a member, and had no power to enforce its decisions.
Offence (such as murder of a civilian or a prisoner of war) that contravenes the internationally accepted laws governing the conduct of wars, particularly the Hague Convention of 1907 and the Geneva Convention of 1949.