North Korea's ballistic missile test on Monday was its ninth this year, the South Korean military says, in an ongoing challenge to new leaders in both the United States and South Korea. A look at key developments:
Jan. 20: Donald Trump inaugurated as U.S. president.
Feb. 12: North Korea tests a new midrange ballistic missile, the Pukguksong-2. It says the missile used solid fuel, an advance that increases a weapon's mobility and makes it harder for outsiders to detect a coming launch.
March 6: North Korea fires four ballistic missiles that fly about 1,000 kilometers (620 miles). Three land in waters near Japan. It later says it was simulating nuclear strikes on U.S. military bases in Japan.
April 5: North Korea fires a missile on the eve of a meeting between Trump and Chinese President Xi Jinping. The missile flies about 60 kilometers (40 miles) before falling into the sea.
April 16: North Korea fires a missile off its east coast, but the launch apparently fails. U.S. officials say the missile was likely a KN-17, a new Scud-type missile.
April 29: South Korean and U.S. officials say a North Korean midrange ballistic missile, presumably a KN-17, failed shortly after launch.
April 30: Trump calls North Korean leader Kim Jong Un "a pretty smart cookie" in a U.S. television interview for being able to hold onto power after taking over at a young age.
May 10: Liberal Moon Jae-in is inaugurated as South Korea's president. He says he wants to improve ties with North Korea.
May 14: North Korea test-fires its newly developed Hwasong-12 missile, which it says can carry a heavy nuclear warhead. Outsiders see the launch as a major step forward because of the height it reached. Moon expresses deep regret over the launch.
May 21: North Korea again tests the solid-fuel Pukguksong-2. The missile flies about 500 kilometers (310 miles) before falling into the ocean.
May 26: Moon's government says it will allow a South Korean civic group to contact North Korea to help fight malaria. It's the first government approval of a cross-border civilian exchange since January 2016.
May 29: South Korean and Japanese officials say North Korea fires a short-range ballistic missile that lands in Japan's maritime economic zone.