The two-day summit has been overshadowed by Trump's decision to order the bombing of nuclear installations in Iran. In 2003, the U.S.-led war on Iraq deeply divided NATO, as France and Germany led opposition to the attack, while Britain and Spain joined the coalition.

The leaders reaffirmed their “ironclad commitment” to NATO’s collective defense clause, Article 5. In recent years, Trump had sowed seeds of doubt about whether the U.S. — NATO’s most powerful member — would come to the aid of any ally under attack.

Trump made the comments to reporters on board Air Force One on Tuesday. Earlier in the day, a White House official said Trump was scheduled to meet Zelenskiy at some point during the NATO summit, taking place on Tuesday and Wednesday in The Hague.