JOHN YANG:

The secretary said, ultimately, it's for Mr. Trump to explain himself, because, as of Friday, he's responsible for that relationship.

We get reaction to Mr. Trump's comments from two people with extensive experience managing U.S.-European relations.

Nicholas Burns was a career diplomat. He was U.S. ambassador to NATO during the Obama administration. He's now at Harvard University. And he joins us tonight from London. Here in studio, Heather Conley, she was deputy assistant secretary of state for European and Eurasian affairs during the George W. Bush administration. She's now director and senior fellow of the Europe Program at the Center for Strategic and International Studies, a think tank in Washington, D.C.

Thank you both for joining us.

Mr. Burns, let me start with you, former ambassador to NATO.

Donald Trump says that NATO is obsolete because it doesn't take on terror, unfair to the United States. What's your take?

NICHOLAS BURNS, Former U.S. Ambassador to NATO: He's completely wrong about that.

I was ambassador to NATO for President George W. Bush actually on 9/11. And on 9/11, when we were attacked from al-Qaida in Afghanistan, the NATO allies came to us in Brussels. They said they wanted to invoke Article V of the NATO treaty, an attack on one of us is an attack on all of us.

They came to our defense big time. They all went into Afghanistan. They bled, died and were wounded for us in Afghanistan. They're all still fighting in Afghanistan. We fought the terrorism of al-Qaida and the terrorism of the Taliban and other terrorist groups in Afghanistan.

So, Trump is — Donald Trump is exactly wrong. And I can tell you, having been in London today, people here are just flabbergasted by this interview in The Times in London.

To basically — to denigrate NATO as obsolete, to root openly for the weakening of the European Union, to castigate Angela Merkel, our strongest friend in Europe, we have not seen an American president be so openly critical of our allies in 70 years. And yet he doesn't criticize our adversary Vladimir Putin.

It is mystifying. People here are uncertain about American leadership. It's a very poor and unwise way to start his term in office.