It does not need to be a coordinated attack - USA alone is enough to scare investors, sure the anarchists will stay with bitcoin till the end - but USA can destroy the exchanges.
mtgox is in Japan. But lets say they shut them down, the volume would be back, distributed among the rest of the platforms within a month, prices in 2 months.
Even if they manage to shut down all the popular exchanges (btc-e is in russia btw, good luck with that) there would be new ones after a week.