Vice President Mike Pence expressed U.S. commitment to Article 5 of the North Atlantic Treaty Organization (NATO) on Monday at an Atlantic Council awards ceremony honoring NATO Secretary General Jens Stoltenberg.

"Make no mistake, our commitment is unwavering," Pence said. "We will meet our obligations to our people to provide for the collective defense of all of our allies....An attack on one of us is an attack on all of us."

The clause has only been triggered once, following the attacks on 9/11.

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"A strong NATO is vitally important, especially in these trying times," Pence said.

Pence's remarks came weeks after President Trump, in his own recent speech before NATO leaders, did not explicitly mention Article 5 and instead called on NATO's European members to spend more on defense.

While he was a presidential candidate, Mr. Trump often talked about reforming NATO.

"NATO was set up at a different time," then-candidate Trump said. "NATO was set up when we were a richer country. We're not a rich country anymore. We're borrowing, we're borrowing all of this money...NATO is costing us a fortune and yes, we're protecting Europe with NATO but we're spending a lot of money. Number one, I think the distribution of costs has to be changed. I think NATO as a concept is good, but it is not as good as it was when it first evolved."

However, on Sunday, Stoltenberg appeared on CBS News' "Face the Nation" and suggested Mr. Trump's criticism had been helpful, in sending a "clear message about the need for increased defense spending across Canada and Europe."

"And a good thing is that the European Allies now understand that we have to invest more in defense, not only to please the United States, but because it is in the interest of Europe to invest more in security because we live in a more dangerous world," Stoltenberg said.