“Scorched Earth” is episode nine of season four of Stargate SG-1.
SG-1 have transported a civilisation to a new planet. They seem to be settling in nicely, when an enormous flying craft over 2 miles wide spouting jets of energy from its underside, scorching everything below, is discovered moving towards them.
The people they have transported there through the stargate are very vulnerable to radiation, and this planet is the only one discovered with an ozone layer thick enough to stop them from going blind – apart from their original homeworld which lacks a stargate and therefore is at an unknown location.
The ship is ‘terraforming’ the planet to suit their own kind. Which requires the destruction of all existing life on the planet.
The alien ship does not want to harm the others, but they can’t stop the process, or their own civilisation will be destroyed. And they need a similar ecosystem to survive.
SG-1 are put in the uncomfortable position of who to let die. Neither civilisation can survive without destroying the other.