Actually, in conventional war, as Millennium Challenge 2002 clearly showed [1], using smart tactics a technologically inferior army can effectively defeat the US military.
No, that only works if the US can't keep coming at you. In that case, you have to be able to sustain massive damage to your economy and military and keep fighting indefinitely (which then guarantees a shift to guerrilla tactics).
This is what I think would happen if the events of MC2002 were real:
If an entire fleet and more than 20000 people were killed in a single day of combat, I bet that the US would have a significant political turmoil, much worse than that of Vietnam, and massive protests. Moving an entire fleet to replace the one lost, could take months, hundreds of billions of taxpayer dollars, loss of morale in troops, and an economic disaster (the markets would simply tank to the lowest).
And of course, moving a fleet to replace the one lost, would mean that you have to lower your guard in another front, which itself introduces new kind of problems. Just as an example, moving the 7th fleet would pave the way for China to invade Taiwan. At the same time, China or Russia would be selling more equipment to the other party.
Right-wing groups would ask for a full retaliation, or more possibly, asking for allies to get involved, but that could rapidly escalate to a regional or global conflict, as other countries would try to support the one playing "David" here.
At that point, with the markets falling apart and clamoring for heads to roll, it would simply be unfeasible to continue.
There's even the possibility that some allied economies (Germany?, France?) would probably refuse to help, as the balance of economy tips in their favor, while US markets tank.
[1] http://en.wikipedia.org/wiki/Millennium_Challenge_2002