The UK is actually the perfect example of this danger. The British Empire didn't dissolve peacefully - it was effectively destroyed by WWI and WWII while trying to suppress a rising Germany.
The subsequent transfer of hegemony to the US was a rare statistical anomaly (a "special case" driven by shared culture and total British exhaustion), but the Empire’s fall itself was catastrophic.
The pattern is violence, not peace. And remember that other aspiring nations to maintain it's position as Empire actively acting to destabilize situation in other states. The reason is simple - it is easiest way to maintain their status.
Brexit for instance was a boon for everybody but UK and EU. There is clear data already about Russian intervention. Recent overt US intervention into ensuring UK remaining separate and EU becoming separate. Think about it.
Of course! Especially because there is no unified army control.
But this requires giving more context. We can't forget that there are ways, especially ways made by empires, to force other nations to go to war not only as an ally but also to make them less relevant and take a hit also.
One of the main factors which makes this more probable, is what op mentioned, the raise of fascism and combatant militaristic attitudes exacerbated by the fact that their own nation / empire is a falling empire. And EU didn't fell yet, it is huge economy with more people than the US.