Question

Why did the increased military expansion in Europe before World War I cause alarm?

Answer 1

See below

Explanation:

Because military expansion usually signifies that a country is planning something. There is no real reason to mobilize an army (as in the case of Russia, for example), unless you are planning to declare war on someone (or defend), because it is rather expensive to maintain a huge army. Therefore, increased expansion in the military = a war is incoming, which obviously alarms nations.