During his campaign, Trump said that the US should only come to the defense of its NATO allies if it can expect those allies to reimburse the US for any incurred costs.

After his nomination as Republican candidate for President, when asked about NATO, Trump said that the US would only intervene if other NATO nations “fulfilled their obligations to us.”

NATO, a mutual defense pact, treats an attack on one member as an attack on all. Only the United States has invoked it, following the 9/11 attacks. During his first NATO summit, Trump simultaneously dedicated a monument to those lost fighting in Afghanistan, including US allies, while also chastising NATO members for not meeting spending targets.

Trump has called NATO “as bad as NAFTA,” an unrelated trade agreement, and suggested that one of its newest members Montenegro could start World War 3.

The New York Times – Donald Trump Sets Conditions for Defending NATO Allies Against Attack
NPR – Casting Doubt On U.S. Commitment To Mutual Defense, Donald Trump Unsettles NATO Allies
The Atlantic – Trump Remains a NATO Skeptic