(SEOUL) — Following U.S. warnings to North Korea of a “massive military response,” South Korea’s military on Monday fired missiles into the sea to simulate an attack on the North’s main nuclear test site a day after Pyongyang detonated its largest ever nuclear test explosion. The heated words from the United States and the military maneuvers in South Korea are becoming familiar responses to North Korea’s rapid, as-yet unchecked pursuit of a viable arsenal of nuclear-tipped missiles that can strike the United States.