It could be argued that the US's (lack of) reaction to Russian occupation of Crimea and its lack of involvement in the situation between Russia and Turkey has eroded the guarantee that NATO is supposed to provide.
Germany though, is a whole different ball game. There are different tiers of countries in the view of the US. Countries like Germany, are in the tier of "we will go to war if you declare war against them". Even if Trump is president. He is right to question the alliances, though.