NATO is viewed as the U.S.'s most important strategic military alliance, including the United Kingdom, France, Germany, Canada, Denmark, and the Netherlands.
"I don't care if I have an agreement or not," Trump said. "The only thing we'd be asking for is what we were asking for before, about 'We want no nuclear.'"
Bush, who traded attacks with Trump as rivals in the 2016 GOP primaries, hailed the president for recognizing the Iranian nuclear threat and acting accordingly.
Those attending the summit were supposed to fly into the airport on Tuesday morning. However, the airport's rail system caught fire and will not be fixed until Tuesday night.