Only with a DDOS against the mining pools(easy), or the miner's computers (harder)
The Bitcoin network has a combined hash rate of 63,841.77 Petaflops ( http://bitcoinwatch.com/ ). Unless they buy a ton of the same kind of specialized chips as the mining community has, they don't have a chance at matching that rate.
What makes you think they won't? According to the calculations upthread, you need about $42M in hardware to hijack the network. You base your entire argument on the fact that the state doesn't have $42M to spend.
BTW: this is why bitcoin is not a currency - the only people who think it's a currency are those not capable of grasping basic logic. The rest of us have no problem acknowledging that it is a barter system based on trust. Barter systems based on trust are okay for small and insignificant communities, like remote tribes in the amazon, early proto-hominids and the like.