Why was the growing of military build up in Europe prior to World War 1 a cause of concern?

Answer 1

As in any case where a country is building up its military strength (like China these days) its neighbors get nervous about where and when all that manpower and hardware is going to be used.

Prior to World War 1, things however were different. Austria, Britain, Russia and France all were woefully unprepared for war. Only Germany was fully prepared (but already had been and remained fully armed since 1870, so for almost 50 years before) . Even its (secret) attack plan dated back to some 10 years before WW 1. You could say that a general military build-up by most countries took place prior to WW 2, but much less so before WW 1.

When for instance Russia finally mobilized in 1914, it had in the beginning about one gun available for every 5 recruits. France was still smarting from its defeat in the war against Prussia in 1870, England had most of its troops committed to its colonial Empire and Austria - the first to start the fight by invading its small neighbor Serbia - had an old-fashioned and badly-led army that was immediately beaten back with heavy losses.

So basically it was only during the Thirties before the start of WW 2 that Hitler's rearmament efforts caused much concern. But until immediately before Germany started the attack in 1914, most countries and diplomats still thought that there would be a diplomatic way out.