PYONGYANG, North Korea — North Korea marked its “Military First” holiday Thursday with mass dancing, outdoor concerts and boasts of a successful – and potentially game-changing – submarine-launched ballistic missile test it hopes will serve as a warning to Washington and Seoul to stop holding joint military exercises Pyongyang sees as a dress rehearsal for invasion.

Television news broadcasts and the front pages of morning newspapers Thursday showed images of the launch, conducted in the early hours the previous day. The test, which brought immediate condemnation from the United States and North Korea’s neighbors, sent a “Pukguksong” missile soaring from a submerged position off the North’s port city of Sinpo. It flew an estimated 310 miles toward the seas around Japan, the longest distance North Korea has yet achieved in a submarine launch.

North Korean leader Kim Jong-un was shown smiling and hugging officials after watching the test from an observation deck. He was quoted by state media as calling it the “success of all successes.”

Launching long-range ballistic missiles from submarines is stealthier than land launching. Having that capability could significantly strengthen North Korea’s ability to conduct strikes on U.S. positions in South Korea, and possibly on U.S. bases in Japan.