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.