Trump's change of heart comes after NATO made multiple appeals to Trump even before the summit started on Tuesday night.

Nearly all members of the transatlantic alliance agree to spend 5% of their gross domestic product on defense — a priority of President Trump's for years.

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.