That stretches it a bit. If a foreign base in your country makes you a protectorate, the USA is a protectorate of Germany (http://en.wikipedia.org/wiki/Holloman_Air_Force_Base#German_...)

In other words: one should nuance that a bit. Yes, in NATO, he USA is by far the biggest player, and it contributes beyond its size, but protectorate stretches it, certainly in some cases.