I think in practice contracts to provide civilian personnel to the Pentagon contain clauses limiting the nature and location of the work - the Pentagon can’t contract for a clerical assistant in DC and then demand they go to Iraq to provide physical security - it violates the nature of the agreed work and the agreed location.

But contracts for personnel generally don’t contain restrictions on use beyond that. If the clerical assistant for DC is asked to provide clerical help to a military planning team who are planning an assault on Chicago, they (and their employer) don’t have legal grounds to refuse. If you are contracted to provide clerical assistance to military planners, you can’t legally say “Baghdad is fine, but Chicago is a no”. Saying that is a breach of contract-unless the courts rule that planning the assault was itself illegal, and I doubt current SCOTUS majority would