The presidents of the US and South Korea have reaffirmed their alliance, agreeing that nuclear armed submarines would resume port visits and threatening a “swift, overwhelming and decisive response” to any North Korean nuclear attack, including retaliation in kind by the US. The South Korean president, Yoon Suk-yeol, said that response would include US nuclear weapons, making explicit an element of the alliance that normally remains unspoken. Yoon and Joe Biden issued a joint declaration marking the 70th anniversary of the alliance, during a visit by the South Korean leader to Washington on Wednesday. “A North Korean nuclear attack against the US or its partners is unacceptable, and would result in the end of whatever regime took the action,” Biden said. “I have absolute authority, and sole authority, to launch a nuclear weapon, but what the declaration means is that we will consult with our allies, if any action is so called for.” The visit and the declaration have come at a time of increasing nervousness in South Korea about North Korea’s nuclear buildup, and calls for Seoul to develop its own nuclear arsenal. The Biden administration is seeking to reassure the South Koreans of US commitment. In the declaration, Biden said that commitment was “ironclad” and that any North Korean nuclear attack on South Korea would be “met with a swift, overwhelming and decisive response”. US extended deterrence, it said, was “backed by the full range of US capabilities, including nuclear”. The declaration also said the allies would consult more extensively “to defend against potential attacks and nuclear use and conduct simulations to inform joint planning efforts”. To underline the presence of the nuclear umbrella, US ballistic submarines armed with nuclear missiles will make port visits in South Korea for the first time since 1991. “Deployment of strategic assets will be made constantly and routinely,” Yoon said. “Nuclear weapons are a source of global insecurity and lie at the heart of the crisis on the Korean Peninsula. Adding more of these weapons to the equation, even temporarily, will not make the United States or South Korea safer or more secure,” said Derek Johnson, managing partner of the Global Zero, a disarmament advocacy group. “This is far more likely to aggravate, rather than alleviate, pressures in the region, which could boil over catastrophically at any moment.”