Monday's ballistic missile test is the ninth such action this year.
Here is a look at the key developments that helped give rise to the latest crisis:
Jan. 20, 2017
Donald Trump inaugurated as U.S. president.
Feb. 12, 2017
North Korea tested a new midrange ballistic missile, the Pukguksong-2. The country said the missile uses solid fuel, an advance that increases a weapon's mobility and makes it harder for outsiders to detect a coming launch.
March 6, 2017
North Korea fired four ballistic missiles that flew about 1,000 kilometers (620 miles). Three landed in waters near Japan. The country later said it was simulating nuclear strikes on U.S. military bases in Japan.
April 5, 2017
North Korea fired a missile on the eve of a meeting between Trump and Chinese President Xi Jinping. The missile flew about 60 kilometers (40 miles) before falling into the sea.
April 16, 2017
North Korea fired a missile off its east coast, but the launch apparently failed. U.S. officials said the missile was likely a KN-17, a new Scud-type missile.
April 29, 2017
South Korean and U.S. officials said a North Korean midrange ballistic missile, presumably a KN-17, failed shortly after launch.
April 30, 2017
Trump called 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, 2017
Liberal Moon Jae-in was inaugurated as South Korea's president. He said he wants to improve ties with North Korea.
May 14, 2017
North Korea test-fired its newly developed Hwasong-12 missile, which it said can carry a heavy nuclear warhead. Outsiders saw the launch as a major step forward because of the height it reached. Moon expresses deep regret over the launch.
May 21, 2017
North Korea again tested the solid-fuel Pukguksong-2. The missile flew about 500 kilometers (310 miles) before falling into the ocean.
May 26, 2017
Moon's government said it will allow a South Korean civic group to contact North Korea to help fight malaria. This is the first government approval of a cross-border civilian exchange since January 2016.
May 29, 2017
South Korean and Japanese officials said North Korea fired a short-range ballistic missile that landed in Japan's maritime economic zone.
June 8, 2017
North Korea fired several missiles, believed to be anti-ship missiles, into the Sea of Japan.
June 23, 2017
North Korea tested a new rocket engine that some believe could be fitted to an intercontinental ballistic missile.
July 4, 2017
The nation's first intercontinental ballistic missile named Hwasong-14 was tested during a launch from Panghyon Aircraft Factory, and landed 37 minutes later into Japan's exclusive economic zone. Authorities said the missile could possible reach Alaska, Hawaii, or even Seattle if it was aiming long.
July 28, 2017
The 14th missile test brought word of North Korea's ability to possibly strike Los Angeles, Denver, Chicago, Boston or New York. The missile landed near the northernmost Japanese island of Hokkaido.
August 8, 2017
North Korea may have successfully produced a miniaturized nuclear warhead that can fit inside its missiles, a Japanese defense paper and U.S. media report said. If the claim is substantiated, it would be a key benchmark in the country's attempt to become a full-fledged nuclear power.
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