Trump had recently announced that he would be ending joint military exercises with South Korea, at the North’s request. During an interview with the Wall Street Journal, Trump was asked if North Korea was trying to drive a wedge between the US and South Korea when he made the remark.

WSJ: You think North Korea is trying to drive a wedge between the two countries, between you and President Moon?

Mr. Trump: I’ll let you know in—within the next 12 months, OK, Mike?

WSJ: Sure.

Mr. Trump: I will let you know. But if I were them I would try. But the difference is I’m president; other people aren’t. And I know more about wedges than any human being that’s ever lived, but I’ll let you know. But I’ll tell you, you know, when you talk about driving a wedge, we also have a thing called trade. And South Korea—brilliantly makes—we have a trade deficit with South Korea of $31 billion a year. That’s a pretty strong bargaining chip to me.

The Wall Street Journal – Transcript of Donald Trump Interview With The Wall Street Journal