NATO Secretary-General Jens Stoltenberg told me he reassured Montenegro after President Trump publicly questioned whether it was worth it for the United States to defend the tiny NATO ally if an enemy attacked. "I have reassured Montenegro," Stoltenberg told me in an interview on Friday, "and I also know that the United States has clearly stated that they are fully committed to Article 5 and NATO and the collective defense."

Why this matters: NATO's foundational principle is that an attack on one member country is an attack on all. America's willingness to honor that commitment provides the security blanket that keeps the transatlantic alliance alive.

But in a July interview, Fox News host Tucker Carlson asked Trump why his son should "go to Montenegro to defend it from attack?"

"I understand what you're saying; I've asked the same question," Trump replied. "You know, Montenegro is a tiny country with very strong people. ... They are very aggressive people. They may get aggressive, and congratulations, you’re in World War III."

Trump was criticized at the time for — perhaps unwittingly — echoing a Vladimir Putin talking point.

I asked Stoltenberg whether he agreed with Trump's assessment that Montenegrins are "very aggressive people."

The NATO leader smiled wryly. "Montenegro is a very welcome ally of NATO," he responded.

"I've been in Montenegro many times ... when I was a child ... we went to Montenegro many times, and I have a good relationship to many Montenegrins."

You can read the rest of my interview with Stoltenberg here. It's a case study in how carefully the NATO leader chooses his words so as not to offend Trump.

The highlights: