Of course they could, but then you'd know it's the US doing it; with cryptographic evidence.
If there are covert operations going on somewhere in the world, where the US isn't publicly claiming to be (and I don't just mean DoD; but also CIA for example); this would be an irrefutable cryptographic trail.
Another case is unintended collateral damage; e.g. say a civilian airliner (with US citizens perhaps) goes down; the US can't have plausible deniability and suggest it was the enemy doing it.
Right but then it would be obviously them vs having the deniability to be like "well it got spoofed by someone who knows who might have interfered in your operation hmmm??"
If there are covert operations going on somewhere in the world, where the US isn't publicly claiming to be (and I don't just mean DoD; but also CIA for example); this would be an irrefutable cryptographic trail.
Another case is unintended collateral damage; e.g. say a civilian airliner (with US citizens perhaps) goes down; the US can't have plausible deniability and suggest it was the enemy doing it.