The inter-connected nature of European diplomacy turning the (sub)continent into a powder keg: if one major player begun a war, the entire territory would (and did) turn into chaos, war and violence. Altho this is a very important cause of the "laying out" of the war, I would argue the most important causes was European's dated military ideals and politics combined with extremely efficient weapon: we now live in a world of very interconnected states, yet no major war has broke out in Europe since WW2 because we know how destructive war can be. The European nations of the 19th-20th century, however, had seen their last war in the 1810's when Napoleon almost conquered Europe, when warfare still was way less destructive, non-industrialized, and had spend these last years conquering technologically inferior military opponent (soon to be colonies). Basically, at the start of World War 1, Europe was under the illusion that war was glorious, almost bloodless, and could be resolved quickly. Turns out modern war is dirty, slow, very costly (both in men and ressources) and is made up of slow, calculated moves that sometime takes years to realize.
Hope that helps!
