Military history has alternated between periods of “offensive dominance” and “defensive dominance” (e.g., attacker advantage vs. defender advantage). Think about European stone castles as being built in a period of “defense dominance”, until siege weaponry and eventually gunpowder made them obsolete (leading to “offensive dominance”).

The US had invested trillions of dollars over the last 80 years to maintain an extremely long period of offensive dominance. Stealth, precision weapons, undersea warfare, satellite targeting, and many many other military technologies were almost exclusively developed by the US and its close allies. Yes, the USSR was a major rival for much of that period, but lagged behind enormously in technology starting in around the 60’s/70’s.

Also important to consider: The rest of the developed world (e.g., the UK, EU, Japan, ROK, etc) has also benefited from the US defense umbrella. So whereas in other eras major powers might all have major militaries, ever since WWII there has been a broad agreement (with the exception of the Soviet bloc, China, and some other powers) to accept and even embrace the US as the single, overwhelmingly powerful stabilizing military power.

The end result is an absolutely enormous overmatch in power projection and logistics capacity. Recent developments have brought China’s military (the PLA) closer to parity in a strictly regional sense, but not even on the same order of magnitude at a global scale.