Operation Terror by Murray Leinster

It was very likely that at that moment Lockley despised himself more bitterly than any other man alive. He blamed himself absolutely for Jill's capture. If there were humans acting with the alien invaders, her fate would unquestionably be more horrible than at the hands of the monsters alone. After all, there was one nation most likely to deal with extra-terrestrial creatures to help them in the conquest of earth, and its troops were not notorious for their kindly behavior to civilians.

And Jill was their captive. He'd been carried past the place where a terror beam blocked the road. The military markings might mean the car was stolen, or that its markings and paint were counterfeit. It seemed certain that Jill had gone up to it in confidence that there could only be American soldiers in such a car, and when near it found out her mistake too late.