The US ambassador to Spain has been summoned by Spanish PM Mariano Rajoy to explain a Snowden leak that shows the NSA is engaged in bulk surveillance of ordinary Spaniards, intercepting some 3.5M calls/day.
If any country were found to be doing this to the USA, it seems likely that elements of the US government would consider it to be a hostile act by a foreign power.
As some have pointed out, the capability for this kind of surveillance will grow steadily cheaper, so you can expect countries like Spain to be in a position to engage in the same conduct with respect to the US in short order -- and now that the US is going around the world, insisting that it is legitimate and even necessary to spy like this, why should they show any restraint?
An NSA graphic, entitled "Spain – last 30 days", reportedly shows the daily flow of phone calls within Spain, and that on one day alone – 11 December 2012 – the NSA monitored more than 3.5m phone calls. It appears that the content of the calls was not monitored but the serial and phone numbers of the handsets used, the locations, sim cards and the duration of the calls were. Emails and other social media were also monitored.
Spain summons US ambassador over claim NSA tracked 60m calls a month [Paul Hamilos/The Guardian]
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.