...“This development would potentially give the DPRK [Democratic People’s Republic of Korea] a relatively secure second-strike capability. This could reduce the value of the U.S. nuclear deterrent against the North Koreans,” Bryan Clark a senior naval analyst at the Center for Strategic and Budgetary Assessments told The National Interest. “That would be significant, particularly in relation to our extended deterrence assurances to Japan and the RoK [Republic of Korea]. If the DPRK were to threaten one of them with nuclear attack, the U.S. could not effectively deter the DPRK by threatening a nuclear attack on it. The DPRK could still plan to launch the strike with its SSBN.”
...Additionally, Benham said that the American commitment to Japan and South Korea is absolute and ironclad, but as Clark noted—the successful North Korean test unavoidably weakens the U.S. extended nuclear deterrence umbrella over those two nations. “Our commitment to the defense of our allies, including the Republic of Korea and Japan, in the face of these threats, is ironclad. We remain prepared to defend ourselves and our allies from any attack or provocation,” Benham said. “We call on North Korea to refrain from actions that further raise tensions in the region and focus instead on taking concrete steps toward fulfilling its commitments and international obligations.
Though the North Korean SLBM will likely reduce the value of America’s extended deterrence over Japan and South Korea, the new weapons won’t be a huge problem for the United States’ own strategic nuclear deterrent. “The SSBN is less of a challenge for U.S. nuclear deterrence. If North Korea were to attack the United States directly, it would not be able to eliminate any leg of the U.S. nuclear triad and the attack could be small enough to be defeated by U.S. missile defenses,” Clark said. “The United States could then launch a devastating retaliatory strike. The fact the DPRK might be able to respond with a small number of SLBMs against the U.S. afterward would not be a deterrent on the U.S. response."
That being said, the North Korean ballistic missile submarines which carry those SLBMs might prove to be a headache for the U.S. Navy’s dwindling attack submarine force, which is set to drop to a low-point of 41 by 2029—well below the required 48 boats (which is already set too low). “The DPRK SSBN may also introduce a new requirement to find and track it with U.S. SSNs, as we did against the Soviets during the Cold War,” Clark said. “The SSBN, however, will likely be fairly noisy and would likely not have the reliability and condition to deploy for long periods far from home. That would make them easier to find and track.”...