Time Crime: Paratime

H. Beam Piper's "Time Crime" is part of his series of alternate history stories which concern the Paratime Police, a law enforcement outfit from a parallel world which has learned how to move between timelines. They jealously guard the secret, even as they mine other worlds for their resources. Notably, it appears that humans are in fact Martian refugees who escaped a calamity on their home planet and migrated to Earth!