North Korea’s nuclear program can be traced to the Korean War, when then-leader Kim Il Sung began taking steps to protect his country through the development of nuclear weapons. Since then, North Korea has routinely conducted missile tests—tests the U.S. has met with a series of condemnations, sanctions and inconsistent dialogue. Tensions have thawed of late under the leadership of Kim Jong Un, resulting in an on-again, off-again meeting between Kim and President Donald Trump. For now, it’s on. And here’s what you need to know.