According to two NATO sources cited by Reuters
Or in their words, he said to have "sharply criticised" the alliance at the NATO summit. A report came up earlier highlighted a threat to withdraw from the alliance completely.
It seems like Trump is really unhappy that other nations are not stepping up to increase defense spending. Either way, it just means that tensions are high and it speaks a lot about the relationship between Europe (Germany) and the US right now.
Update: It is said that Trump significantly toughened the tone and singled out Germany and Spain among others in earlier talks.
