How did militarism help cause World War I?

The unification and rapid militarization of Germany after 1870 was the factor that destabilized Europe in the latter part of the nineteenth and early twentieth century, leading to a chain reaction that eventually sparked World War I, a war that outdid all others up until its time in its violence and loss of life.
It is easy to blame Germany for its aggressive and destabilizing militarization, but the newly unified nation was, from its point of view, merely trying to catch up with states like Great Britain and France that had had a huge head start in developing into imperial powers. Germany wanted an empire like those it perceived as enriching rival European nations, and it wanted it fast. It believed that a ramped-up military was the best way to achieve this goal. It also wished to be the dominant power on the European continent and built up its armies with that idea in mind.
After its victory in the Franco-Prussian war, Germany was eager as well to take on the dominant world superpower, Great Britain, especially for control of the seas. Great Britain was beginning to weaken, but, from its point of view, no way was it about to give up its naval dominance. As Germany massively built up its fleet, so did Great Britain, heightening tensions across the continent.
Because both major and minor European nations were on edge about the possibility of war, they entered into a series of alliances in which they promised to support each other in case of attack.
The assassination of Archduke Franz Ferdinand was the match thrown on the fire that blew up the whole edifice. Because the increased militarization of Germany had led to a complicated chain of alliances, almost of all of Europe got dragged into a war that might have been contained. It is easy—and to some extent, right—to blame Germany for this, but they also were copying their neighbors's playbooks, and there is plenty of blame to spread around on all sides.

Militarism was the predominant historical belief that a sovereign nation should house a vast and powerful army to defend itself from any potential threats. Consider Nazi germany after World War I was concluded. One of the main things Hitler did was to reestablish the army because he believed that they should be able to defend themselves, but also because most nations considered it a right of theirs.
This military might led to nations that felt nearly invincible if they went to war. Because of this sentiment, the nations were far more willing to engage in combat because they felt like they would be safe, even though every other nation had just as mighty armies. In addition to contributing to the start of the war, it certainly meant that World War I was one of the most destructive and deadly wars ever on the face of the Earth.

While there is no singular cause of WWI—it was a complex combination of many factors, including nationalism, international competition, and ethnic tensions in the Balkans—militarism played a large role in bringing about not only the war, but also its legacy as a truly modern and brutal war. Militarism refers to the belief that a state or nation should maintain a large, strong military to not only defend itself, but to also expand or carry out its own interests. In the years leading up to World War I, the major European powers focused on expanding their standing armies and navies, and competing with one another to build the largest, strongest, and most technologically advanced military.
Much like the arms race during the Cold War, this early arms race increased tensions and competition between the European powers, especially Britain, France, and Germany. Military preparedness became the central focus not only of a country's government and politics, but also of its society and culture. Young men were drafted into the army, and officers were often members of high social classes who attained social status. Realizing the dangers of military buildup, the major European powers held the Second Hague Conference in 1907, but Germany opposed disarmament. Instead, Germany's navy began catching up to Britain's navy, which had been the largest and strongest in the world. By the time of WWI, large standing armies were fully equipped with revolutionary new ships (Dreadnoughts), mustard gas, tanks, and machine guns.