The president made the claim in an unscheduled press conference Thursday and said he told allies, "I would be very unhappy if they didn't up their commitment very substantially."
The summit is an opportunity to present an updated strategy for the transatlantic alliance, but President Trump is unlikely to follow the script.
Trump boasted that the NATO leadership is very fond of him, but says Russia must be thinking "if we did like this guy, we made a big mistake."
The letters said America's contribution to the NATO alliance is "no longer sustainable."
After two years of NATO's defense building, should the Kremlin be worried?
The Ministry of Defense wants a new center to coordinate Baltic deployments.
Two aircraft carriers will be in Mediterranean as Washington seeks to balance Russia's military activities and accelerate its fight against ISIS.
Poland’s defense minister has confirmed discussions with NATO’s Secretary General.
President Obama made the remarks at the NATO summit in Newport, Wales