They don't need to shut down the network to win, just like you don't need to destroy an adversary's weapons to win a battle. You can win by destroying his will to fight. In this case, scare bitcoin users into abandoning the currency. Some high-profile prison terms would ensure that.
This has worked really well in the war on drugs, I.e. three strikes law. It's pretty impractical and extremely expensive to fill up your prisons with bitcoin holders.
Actually, they would probably only need to raid a substantial amount (maybe half?) in order to control the block chain.