The US wouldnt stand a chance.

UNSC would have spacial superiority, meaning no fancy satelites to transmit pictures or communication.

I dont know much about computers, but i bet that a couple of hundred smart A.I.s could hack their entire defence network,.

the predator drones someone have mentioned would be useless, because the UNSC could just take control of the uplink.

Not to mention the vastly superior numbers of the UNSC.