I’ve never heard the “short century” thing. I take it the brackets are WW1 and the *real* end of WW2 - the fall of the USSR. I’ll have to think about that.
I suppose it follows from the “long century” that I’ve heard historians discuss, namely 1789 (start of French Revolution) to 1914 (start of WWII). That makes sense as those dates bracket a distinct era of European history. The French Revolutionary/Napoleonic wars led to an era that was essentially void of major great power conflicts,(Franco-Prussian war and Crimean War were relatively minor skirmishes), an era that came crashing to a halt in 1914.