Yeah it wouldn't be worth it for most countries.
You need a big enough population to "hold" it....so really only China, India etc., got that.
Then you gotta be able to handle the U.S. Navy and ain't nobody doing that.
In the air? Nah, still getting washed.
Next thing is that Americans are probably the most well-armed population of civilians in the world. We got enough guns that damn near everyone ages 18+ could have at least one or two plus plenty of ammo.
Then you gotta factor that the country isn't densely populated. You could take our New York or LA but that's not going to bring the whole country down. If you gotta put boots on the ground....good luck. It's why we're such azzholes to the world because we know an outside force can't really touch us. Our issue is internal politics and racist wealthy whites trying to burn it all down for a few extra dollars.