>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.