Basic plot of "The War Games" is that these aliens are using Time Lord technology (thanks to another renegade Time Lord) to kidnap and brainwash soldiers from earth throughout history so that they can basically take over the galaxy, using the logic that "Humans are the most vicious species ever". There's even discussion of dropping a neutron bomb on rebels that are trying to overthrow them. The Doctor eventually calls for the Time Lords, who force the Doctor to regenerate without his consent, while conscious.
