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In the late 19th century, powers across Europe began rapidly expanding their armies. This became known as "the arms race." During this time, Britain built a navy that was twice as strong as France and Germany's combined, Germany attempted to expand its navy to compete, mobilization systems were developed to call men to war quickly and Germany, Russia and France developed systems with millions of reservists. As Europe was coming towards the end of a century long peace time, this made the respective powers suspicious and caused tension.
One of the most important aspects of militarism before WWI was the Anglo-German naval race. While Germany attempted to build a navy that could eclipse Britain's, Britain pushed to develop its forces to avoid this. When Germany started making progress, Britain entered the Triple Entente alliance, which was made with Russia and France. As Russia had respective alliances with other countries, its response to Austria invading Serbia in 1914 dragged Britain and France in with them.