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As the world worries about the increasing threat from North Korea’s dangerous leader Kim Jong Un, there is a temptation to believe that the problems will be solved ashore. South Korea is exploring a return to the so-called sunshine policy of using trade and engagement incentives to defuse the situation. The U.S. wants to deploy a new ground-based missile defense system. China is increasing its control over the notoriously leaky border.

All of these are prudent moves, but it is worth remembering that Korea is a long peninsula, with approaches dominated by the sea. So it is perhaps not surprising that China and Russia are pushing vigorously to stop the robust program of at-sea exercises the U.S. undertakes with allies in the region as a precondition for talks with North Korea.

While canceling some practice drills off the Korean coast may seem like no big deal, doing so would be one of the worst mistakes the U.S. could make.

It is important to understand just how vital it is to train at sea, in the demanding maritime environment itself, if we are to be prepared to operate effectively in a crisis. Sailors and their ships effectively rust in port, and the complexity of large-scale activities on the oceans demands weeks of training individually for each ship, followed by weeks of working together.

The recent collision of the U.S. destroyer Fitzgerald with a container ship off the coast of Japan reminds us how inherently difficult it is to operate on the high seas. Our Navy ships typically train for over a year in U.S. waters before deploying forward. Working with other navies — given language, cultural and operational differences — adds another layer of difficulty.

There are five key maritime tasks ahead of the U.S. military.

First, as the Trump administration seeks to control escalation and find a way for diplomacy to succeed, we need to gather intelligence effectively. That work can best be done from international waters by the U.S. Navy’s stealthy submarines and Arleigh Burke-class destroyers equipped with Aegis combat systems.

Second, sea power is also vital for enforcing sanctions against North Korea, including those passed unanimously last week by the U.N. Security Council. Over 90 percent of non-Chinese trade for North Korea moves across the oceans, and our ability to stop it matters deeply.

Another crucial element of deterrence is ballistic-missile defense. The ability to use sea-based platforms — the Aegis-equipped destroyers and the comparable Ticonderoga-class cruisers — in the waters off the southern portion of the peninsula is central to neutralizing North Korea’s long-range missiles. Coupled with the land-based system known as THAAD, these maritime systems can significantly reduce the power and lethality of the North Korean threat.

Finally, if we do end up applying “hard power” — a very bad option, but certainly a possibility — much of it will come from the sea. A strike force consisting of up to three carrier groups would bring together hundreds of tactical strike aircraft, as well as hundreds of Tomahawk cruise missiles. Supplemented by long-range Air Force assets, this armada would be crucial in striking the North Korean leadership and nuclear program. Given the ships’ ability to move nearly a thousand miles daily, it could operate on both sides of the Korea peninsula — out of range of effective North Korean countermeasures — and continue to pummel the regime.

One final consideration may seem counterintuitive, but it is worth considering — doing some level of exercises with China itself. These would best focus on “benign” operations such as training in disaster relief, medical diplomacy, humanitarian projects, narcotics interdiction and the like. While not operationally significant, such exercises would help build confidence in terms of being able to communicate rapidly with the Chinese navy in times of crisis.

Given all of the maritime capability that is needed in dealing with North Korea, it is clear that the U.S. must exercise on the high seas and practice all of the scenarios outlined above. These exercises cannot be a bargaining chip in multilateral negotiations.

While Kim likes to maintain an air of bravado, he is not irrational — he knows open combat means the end of his nation and his family. Making clear to him the military threat he faces from the sea will help us prepare for the worst and, more important, perhaps help coax him to a negotiated conclusion.

James Stavridis, a retired U.S. Navy admiral and former military commander of NATO, is dean of the Fletcher School of Law and Diplomacy at Tufts University and author of “Sea Power: The History and Geopolitics of the World’s Oceans.” He wrote this article for Bloomberg View.