What happened: North Korean leader Kim Jong-un announced the country will stop conducting nuclear tests and launches of intercontinental ballistic missiles starting April 21, and shut down a nuclear test site in the north side of the country. Per BBC, Kim said further tests were unnecessary because Pyongyang's nuclear capabilities had been 'verified.'

Trump tweets: "This is very good news for North Korea and the World - big progress! Look forward to our Summit. ... Progress being made for all!"

Be smart, from a former National Security Council official under President George W. Bush: "Not a big deal to pause testing nuclear or missiles when summit is coming ... Totally reversible. Eye wash."

Victor Cha — senior adviser and Korea Chair at the Center for Strategic and International Studies, who was Trump's candidate for ambassador to South Korea — tells Axios future editor Steve LeVine that the shift isn't as abrupt as it looks:

• "They have already stated that they would halt all testing while in dialogue. This statement formalizes that promise."
• "[T]he statement talks about a test ban, no first use, and no transfer — all the trappings of a 'responsible' nuclear weapons state (which is what they ultimately wanted to be accepted as)."
• "Thus, this is not a denuclearization statement, it is a statement that DPRK [North Korea] can be a responsible nuclear weapons state."
• The key sentence: "No one believes this, but if they can get Trump to agree, that is all they need."

What to watch for, per Cha: "[T]he glaring unanswered question in all of this is what the U.S. will give in return for these up front N.K. concessions."

• "We know what we want from DPRK, but no one is scrutinizing what the US administration is thinking of giving up — sanctions? peace treaty? normalization? exercises? missile defense?"