The United States is continuing to up the ante in its confrontation with North Korea.

Even as U.S. Air Force B-1B Lancer strategic bombers practice overhead with the Japan Air Self-Defense Force and the Republic of Korea Air Force, lurking underneath the waves are cruise-missile laden U.S. Navy nuclear-powered attack submarines. While the vessels are normally hidden from view, the U.S. Navy recently allowed the Los Angeles-class (SSN-688) attack submarine USS Tucson (SSN 770) to visit US. Fleet Activities Chinhae in South Korea on October 7.

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"The Korean-American relationship is very important and our visit to Chinhae gives us the opportunity to strengthen the outstanding relationship that exists between the U.S. and the Republic of Korea," Cmdr. Chad Hardt, Tucson’s commanding officer said. "My crew and I are looking forward to experiencing the exciting culture of this great Korean city."

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Tucson’s visit is a signal to the North Koreans that while Pyongyang might not always be able to see American forces, the U.S. military can bring a significant amount of long-range precision strike capability to bear very rapidly. While Tucson’s visit is a signal to the North Koreas, the attack submarine’s visit to South Korea at a time of heightened tensions with a nuclear-armed Pyongyang is meant to reassure Seoul that the United States won’t abandon its allies.

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Tucson is not one of the top of the line Seawolf-class (of which only three were built due to their sheer expense) or new Virginia-class submarines (of which only 15 have been completed), rather the vessel is part of the long-serving Los Angeles-class, which still makes up the bulk of roughly 52 SSNs in the U.S. Navy fleet. Indeed, Tucson is the 59th 688-class boat to be built and was the 20th Improved Los Angeles submarine to be completed. Though no longer representing the state of the art for attack submarine design, Tucson and the other 688-class boats are the workhorses of the fleet.

Improved Los Angeles-class submarines like Tucson—like the later Virginia-class—are equipped with twelve vertical launch tubes to carry Tomahawk cruise missiles. Unlike many other U.S. military assets, submarines–which are some of the stealthiest platforms anywhere—can close in to launch their cruise missiles with effectively no warning. As such, a weapon such as an attack submarine can keep the enemy guessing as to where a missile strike will come from.

In the case of North Korea, Tucson’s presence in the waters off the Korean peninsula serves as a warning to Pyongyang to moderate its behavior.

Dave Majumdar is the defense editor for the National Interest. You can follow him on Twitter: @davemajumdar.