There’s more to taking over the Earth than simply planting your flag in its soil and telling a bunch of Earthling bystanders that you claim the planet, plus its moons and satellites, for the greater glory of your intergalactic empire.
Commander Linx (Kevin Lindsay) of the Fifth Sontaran Space Fleet learned this to his cost, after crash-landing in Wessex circa 1200 AD (in The Time Warrior, 1973-74). For one thing, Earth’s revolting natives – in this instance, a robber baron named Irongron (David Daker) and his assembly of cut-throats – have a habit of revolting against you, sooner or later. For another, you might find even such a limited incursion halted by the intervention of a Time Lord who sometimes claims to be the planet’s protector. No, if you’re going to invade the planet Earth, you can’t do it on the fly. You’re going to need a plan…