As of now the US have the greatest army, but if let’s say China were to prepare for it for enough time in a somewhat stealthy way, couldn’t it bring the USA into a long and dirty foght ?
Well, this is one of the reasons a Great War is unlikely - how are you going to do this? Even if you build things up slowly, you still have to store them somewhere. You have to train people. There is no way to create a military force capable of fighting a Great War in today's age without it being obvious.
Which isn't to say that build up can't happen - it's just not going to be a surprise to anyone.
Trained via software updates and built under the guise of something innocent like 'gardener bots'
Force extended requires significant amplification.
So for China to impact us in a meaningful way they would have to somehow sneak a force large enough to damage across a pretty wide open space, then maintain that logistically.
Ditto for the RU
That said once in the US you have 300 million small arms manned by 21+ million veterans in the US
Not sure China can actually get that many boots on the US soil before the Navy / Air get a poke or two at them
Russia is only 55 miles from the US.