The answer is as simple and obvious as the nose on your face -- even though many can't seem to see it.
Money.
When Pres. Jimmy Carter gave away the store (i.e., Panama) and nearly lost the Cold War, he had reduced defense spending to a mere 4.5% of GDP, down from nearly 10% during the Vietnam War.
Pres. Ronald Reagan won the Cold War by increasing defense to 6% of GDP.
Today, Pres. Trump tells the Euros they must increase their defense from 2% to 5% of their GDPs.
Today, under Pres. Biden, US defense spending is around 3.3% of US GDP.
Under Pres. Trump, the only word I heard from Sec. Hegseth is that he intends to reduce that to 3% of GDP, while somehow increasing "lethality".
Of course, pretty much any military can be adequate, so long as it never has to deter or fight a war...
We'll see.
I remember seeing a retired general years ago on Johnny Carson saying that we could greatly reduce cost by relying primarily on nuclear weapons. For example, after 9/11, just nuke Kabul. No carriers, no troops, and certainly no nation building.