It's the problem of history, really; redrawing Europes map in the wake of the Napoleonic Wars, and a century of dancing around the consequences to avoid another major war, made some form of the Great War inevitable. The pressure was never relieved, it just built with each attempt to contain it, until it exploded.

Without the Great War the tensions that led to the Bolshevik Revolution are still present; they'd been festering in Russia for some time before 1914, already having resulted in the assassination of a Czar and the exile of Lenin well before war broke out in Europe. That comes to a head sooner or later, too; the absence of the Great War might mean the White Army wins the Russian civil war due to greater Western aid, but the problems remain, and the fact is Russia was plenty autocratic even before Stalin, so the same Cold War might have resulted, only slightly mitigated by the debt the Czars owed the west for keeping them on the throne. And, yeah, nuclear weapons are just the inevitable march of science; had America not used them it's entirely possible WWII would've lasted long enough for Japan to develop them, a big game changer.