Either directly or indirectly, as per the following examples:
If a contractor fails to carry out the duties and responsibilities explicit in the contract (e.g. by failing to provide security when obliged to do so, or by doing so inadequately) then, as with any other business contract, the contractor has voided his contract.
However that contractor also has a duty to comply with human rights legislation, and with all other relevant legislation (even if no such duty is explicit in, or even implied by, the written contract). So any action that breaches human rights legislation, or the laws of the relevant country, may also be deemed to void the contract.
If you require a more detailed answer you'll probably need to provide further information about exactly what it is that you're seeking. In the meantime, this is worth reading:
https://lapa.princeton.edu/conferences/military07/restricted/dickinson2_military07.pdf