The Korean War is apparently back on, and North Korea is moving military assets, evidently to confront American warplanes flying along its eastern coast. At the same time, South Korea has announced that the U.S. will soon be repositioning “strategic assets” closer to the Korean Peninsula.

Both sides, in short, are in the early stages of mobilization.

On Wednesday, Chung Eui-yong, chief of South Korea’s National Security Office, disclosed that the U.S. had promised in writing to move forces to support the South. “The U.S. has pledged to expand the rotational deployment of its strategic assets near the Korean Peninsula,” Chung said during a meeting convened by President Moon Jae-in in his underground crisis-management center. The Pentagon, Chung said, “will begin as early as late this year, and this will help us expand our defense capabilities.”

Chung did not detail which U.S. assets he was referring to, although the phrase he used is generally understood to mean nuclear weapons.

Chung’s startling disclosure followed a Reuters report from Monday in which a “U.S. official,” speaking anonymously, said satellite imagery showed North Korea had moved a few aircraft to eastern coastal areas.

Nobody would have paid attention to the news—Reuters would not even have reported it in all probability—had not the same day North Korea’s foreign minister, Ri Yong Ho, made an attention-grabbing statement of his own.

“Last weekend Trump claimed that our leadership wouldn’t be around much longer and declared a war on our country,” Ri said at an impromptu press gathering in New York. “Since the United States declared war on our country, we will have every right to make all self-defensive countermeasures, including the right to shoot down the United States strategic bombers at any time even when they are not yet inside the aerospace border of our country.”

Ri’s provocative declaration followed his Saturday speech before the U.N. General Assembly in which he said that Trump’s words were making a North Korean strike on “the entire U.S. mainland inevitable.”

The foreign minister’s comment was a prediction, not qualified with “ifs” about a preceding American attack.

In the past, the North Koreans often made belligerent statements about incinerating U.S. cities, like Austin or Washington, but they were almost always careful to put their words in the context of retaliating for a previous American strike on their soil.

That caution has been less and less evident during the last week. Ri’s General Assembly “inevitable” comment was, in substance, a threat to strike first—as was his declaration about shooting down American planes in international airspace.

Monday’s shoot-down threat was made just after the Pentagon announced that on Saturday B-1B bombers, flying from Andersen Air Force Base on Guam and accompanied by F-15C fighters from Kadena on Okinawa, flew near the North. “This is the farthest north of the Demilitarized Zone any U.S. fighter or bomber aircraft has flown off North Korea’s coast in the 21st century,” noted Defense Department spokeswoman Dana White in a statement.

And Ri’s threat to take American planes out of the sky was another abrogation of the July 1953 armistice that ended the fighting in the Korean War. His words showed that the pact, an agreement not to use force, no longer exists.

At least three times in the last decade—2003, 2006, and 2009—the North Koreans renounced the armistice. The U.N. Command, an armistice signatory, takes the position that the agreement is still in force. That position, dictated by Washington, is untenable as a matter of law because one party has repeatedly signaled, both by word and conduct, that it is no longer bound.

Ri’s words can be extremely useful for the U.S., however. With no agreement not to use force, the U.S. has the legal authority to conduct military operations against the North. For instance, the U.S. Navy now has the authority, specifically denied to it by Security Council Resolution 2375, to board North Korean ships to conduct weapons searches.

The U.S. can also enforce, with its military, President Trump’s sweeping Sept. 21 executive order, which imposes sanctions on foreign banks and businesses maintaining trade and other economic relations with the North.

Defense Secretary Jim Mattis, on Sept. 18 cryptically said the U.S. has military options that do not put Seoul in grave jeopardy. He refused to say what those options were, but the boarding of ships is likely to be one of them.

The boarding of ships, as dangerous as it can be, looks increasingly necessary. For one thing, the U.S. cannot continue to allow the North Koreans to sell, among other things, missiles to Iran and chemical weapons to Syria.

For another thing, it is high time that the Trump administration unnerve the regime of Kim Jong Un. For far too long, the North’s leader has felt he can, with impunity, make threats, launch cyberattacks, fire off missiles, and detonate nukes. If the international community wants to disabuse him of the notion that he can do what he wants, the U.S. will have to impose severe costs.

So far, the North Koreans do not seem to be deterred from dangerous conduct. As Scott Snyder of the Council on Foreign Relations wrote recently, “Kim’s default assumption is that the U.S., at the center of the existing international order, is fundamentally constrained in its ability to respond to North Korea’s nuclear advancement.”

Feeling constrained, the U.S. has allowed the Kim family, over the course of decades, to get away with murder, literally and figuratively. That cautious American approach, which many have praised as responsible, has unfortunately not led to good outcomes.

And the situation today can deteriorate fast. After all, the North Koreans and Americans are each beginning to move their militaries closer to the other.