After the cold war has ended, the western nations focused on not giving a fuck about military strength, allies or facing opponents the size of Russia or China. Instead some small-time infantry campaigns like Iraq, Syria and Afghanistan were started and then basically abandoned without any decisive result or even with a total failure. But even back in the 2000s, there were no common goals and no real alliances, just some commitments to save face and keep up appearances.
So now, Russia in Ukraine and Iran in the Persian Gulf have called the bluff that is Western military power and decisiveness. I'm guessing those were just the first instances of a long line of wars that should have been prevented by the US-lead West, that are now possible because the West is fractured and aimless.
Interesting choices, to frame the US-Iran war in the same terms as the Russian invasion of Ukraine, as if Iran was the aggressor, and to frame the invasions ("small-time infantry campaigns") of Iraq and Afghanistan, which led to millions of lives lost, in the same terms as the comparatively minor US intervention in Syria.
The common thing is that there were no desired results because the west decided to do some half-assed thing and just go with the minimum amount of force and troops. Also, some parts of the west decided to start/intervene/participate, while others abstained. In short, nobody cared enough about the goals and about their respective western allies to go in with a heavy force, accept some losses and do the job properly.
Of course circumstances, reasons, participants and histories beyond that were different.
Hopefully, Europe has learned its lesson. The way it was handled post-WW2 is pointing in the right direction.
WW2 was the direct result of that "historically consistent belligerence". Simplified history: lots of tension in Europe -> WW1 -> screw you Germany, we won -> Hitler: we are not having it! -> Germans: that's the guy we need! -> Oops -> WW2.
That why they did it differently post-WW2, focusing more on international collaboration (and a little indoctrination, you don't want to miss an opportunity do you?).
Also, Europe is not the center of the world anymore, they have better things to do than infighting.
The last 80 years have been exceptionally peaceful compared to the rest of history, so I hope it continues despite America's loss of hegemon status.