The First World War” The MAIN cause that led to World War I Militarism (a race to build bigger and better armies and fleets than other countries) Submarines, tanks, gas, machine guns, and aeroplanes were some of the weapons that European countries had that made war deadlier. Alliances are deals between groups of people to help each other out in times of war. Alliances were made... https://worldswar3.com/the-first-world-war-1914-1919/