Don't be ridiculous. The US alone cannot stand up to China. We can't even build ships anymore. Parity is only possible with an alliance comprising the full economic and manufacturing might of the "free" developed world, which includes Europe and the rest of Asia. Losing Europe as a close military and trading ally is suicidal all by itself, but it has knock-on effects: once you've abandoned Europe, do you think the rest of your allies are going to believe you're there for them? Why should South Korea risk its security building ships for a US military alliance, once we've signaled that the US values its military alliances at zero?