The mercenaries need to demonstrate they would have made a good-faith attempt at the mission objectives had they not been hamstrung, but rules of engagement preclude offensive operations. The mercenaries are allowed to support patrols that come under attack, however, so with a little creativity they may have a pretext to pick a fight and hopefully win it. An unexpectedly hot engagement reveals the insurgents are receiving offworld support, including advanced weapons. The mercenaries have been ordered to withdraw, but the insurgents base could be a threat to their retiring force. They are allowed to clear their flanks, but that would require raiding the insurgents' home base, defeating their forces and fulfilling the contract despite the clients intentions. And if they happen to seize a shipment of powerful weapons in the process, well, that is mere coincidence.