Spying on their "allies"

On Wednesday Germany uncovered a second American spy in its military, and retaliated today by evicting the local CIA station chief. I'm wondering now whether we'll see Cold War-style tit-for-tat expulsions between two ostensible "allies".

Meanwhile, the US infiltration of its ally Germany raises the obvious question: are they spying on us? And what measures, if any, is the government taking to prevent it?