North Korea's in a good mood these days.
The nation is celebrating its first successful test of a submarine-launched missile.
The country's leader — Kim Jong Un — says the US mainland is now within striking range of his nuclear weapons.
That sounds like a threat.
Joel Wit, a former US nuclear negotiator with North Korea, says he's concerned, but not worried. "Because — despite this success — we’re not within striking range of their nuclear weapons."