Sen. Dan Sullivan (R-AK) on Tuesday evening warned that President Donald Trump would need approval from Congress if he wanted to preemptively go to war against North Korea.

“If one of the military options that the administration is looking at is a preemptive war on the Korean peninsula launched by the United States, that would require the authorization of Congress,” Sullivan said on Fox News. “Article I of the U.S. Constitution is very clear about that.”

But he noted that if North Korea acted against the U.S. first, Trump would not need Congress’ go-ahead to hit back.

“Obviously, as the commander in chief, the President can react to attacks on the country in a way that he has broader authority on that. I was mentioning the discussions of a preemptive war on the peninsula, that clearly goes in the realm of the authorization of Congress,” he said.

Sullivan’s comments on Fox News followed remarks Trump gave Tuesday afternoon at his golf course in Bedminster, New Jersey, warning North Korea against using a nuclear weapon.

“They will be met with fire and fury like the world has never seen,” Trump said. “He has been very threatening, beyond a normal state, and as I said they will be met with fire and fury and, frankly, power the likes of which this world has never seen before.”

The President followed up on Wednesday by touting the United States’ nuclear arsenal in a series of tweets.