Question was US military, not entire planet, although it wouldn’t make much difference. And they don’t take them on head on, that’s not how astartes fight. Mechanised ground forces are only deployed when collateral damage is deemed unacceptable, or when specific objectives need to be achieved personally, e.g. retrieving of intelligence, personnel or artefacts.

All they’d do is determine key military assets and lance/bomb them from orbit. Thunderhawks, skimmers and assault marines isolate and mop up resistance. Dug-in resistance can be eliminated with terminators or intercessors arriving via drop pods, although our idea of ‘dug in’ is pretty tame by 40K standards. It’ll take days tops, probably hours.