This is likely just a demonstration of US resolve to protect South Korea because I cannot believe anyone is taking the North’s threats of launching a nuclear attack seriously:
After two Korea-U.S. joint military drills end, American vessels equipped with nuclear weapons will stay in South Korean waters to fully guarantee the U.S. “nuclear umbrella” in case North Korea attacks.
A high-ranking South Korean government official told the JoongAng Ilbo yesterday, “If North Korea makes a nuclear attack, retaliation can come from U.S. nuclear weapons stationed in Okinawa or Guam. But considering the time that might take, we need to have a nuclear weapon near the Korean Peninsula.
“By not withdrawing U.S. weapons participating in the Korea-U.S. military exercises, we decided to let them stay a while and see what happens in North Korea,” he said. [Joong Ang Ilbo]