North Korea is showing no signs of slowing down. On Sunday, it fired its third missile test in three weeks.
The rocket touched down about 230 miles off the Japanese coast where commercial ships often sail.
Japan's prime minister promised "concrete action" after the launch, and South Korean military officials said the North would get a "strong punishment."
North Korea has tested this type of short-range missile before, but experts say they believe the data from this launch could be used to develop an intercontinental ballistic missile.
And in preparation for that, the U.S. is set to test missile defense technology on Tuesday.
The tech's official name is the Ground-Based Midcourse Defense system. When there's an incoming missile, the system shoots its own so the two collide and explode over open water. However, the system's reliability has been the main worry.
In an interview that aired the same day as the missile test, Defense Secretary James Mattis said the U.S. should respond before North Korea develops long-range missiles.
But he also warned if a diplomatic solution isn't found, war with North Korea would be "catastrophic."
Mattis told CBS, "The North Korean regime has hundreds of artillery cannons and rocket launchers in range of one of the most densely populated cities on Earth, which is the capital of South Korea."
For that reason, he said, "Conflict in North Korea ... would probably be the worst kind of fighting in most people's lifetimes."