How did militarism help to cause World War I?

Starting around the turn of the twentieth century, Germany engaged in an increasingly aggressive naval buildup meant to challenge Great Britain's dominance of the seas. Since the defeat of Napoleon in 1815, Britain had been the undisputed ruler of the world's waterways. Britain was the single superpower through most of the nineteenth century, and its powerful navy was unrivaled.
Britain naturally reacted with suspicion and unease to Germany's naval buildup. Germany had unified in 1870 and, since that time, had become a formidable power. It wanted, belatedly, to participate more fully in the spoils of imperialism and felt it needed a strong naval presence to challenge British dominance.
Both countries engaged in a naval arms race in the first decades of the twentieth century. Britain became especially nervous when Russia lost its naval power with its defeat in the Russo-Japanese war. In both England and Germany, enthusiasm for military buildup built nationalist tensions.
In a realigning world, increased militarism simply fueled the fires of suspicion between European nations, which culminated in a destructive world war.