Did the US ever use militarized robots in combat?

only stay in bases

Which is what made the original robots make sense.
The military buys a bunch of robots that don't quite live up to the marketing hype, and hands them over to you.
Not trusting them to do much, you let them focus on menial tasks that take the burden off your staff. Simple repairs, guarding doors, etc. Which is why the Enclave has them, but they just hang around bases.
In Fallout 2, pre-war combat robots are peaking with the Robobrain, which is still not widely trusted as a combat unit. Bipedal travel is still too difficult to figure out, so they rely on tracks.
In Fallout 4, pre-war combat robots have retroactively evolved to Assaultrons - bipedal, autonomous super-assassins with deadly close and ranged combat options, as well as EMP shielding. If a combat robot had written Fallout 4, Assaultrons would be called Mary Sues.

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