There are lots of our colonial bretheren stationed in South Korea. If there were not, the North would have attacked by now and the job be jobbed. Because there are, the North know, with certainty, that the USA would consider an attack on the South as an attack on the US and it would be met with violent retribution. In essence, the US servicemen in S.Korea are pawns in a game, a human shield if you will, not there to stop the North physically but to send a clear message.
One does wonder what they'd gain from starting anything... I can't see anything in it from them. Though I suppose that Yung-un has probably been fed so much bullshit by now from people only telling him what he wants to hear that he thinks DPRK could take on the US all by themselves.
But if it did kick off, I don't think it'd take long before a humiliating defeat.