Anything out of hundreds of possibilities.
But it is not how did it happen and the cause, although it is important, but why did it happen and what was the goal.
On a grand scale, dwell into conspiracy thinking - goal is total control of the nations and planet resources, by whoever is behind the goal of total control. After great war as was ww1, we did get international/multinational institutions, and after ww2 as well.
War by war by war, and as more bloody as possible, will sooner or later smash the will of mankind after seeing such horrors, that people will surrender and unify or bend under anything that offers peace.
In this case we talk about europe, and peace in europe has not existed for thousand or so years, battle hardened nations for most part. Short patches of peace 20-50 years in span, few lucky generations avoided it. So they needed as fucked up war as possible, and whoever they are, they made sure that local conflict escalates into world war. WW2 was rinse and repeat.
We are still awaiting for ww3 since there is no total control of the planet, right?
And watching the news we can see forces are at work trying as best as they can to get Usa Vs Russia to go off and fuck us common folk in the process.
I do not think for a second, that in arena of international politics and even domestic, anything happens by chance.
They planning horrors for us while we work, fuck, eat and go to sleep, day after day.