Clever technology means they can appear anywhere they like at will. No home is safe. No inhabitant safe from being plucked out or destroyed. They say they are on a peace mission and talk of this organisation they are part of that has the co-operation of other planets like yourselves. Even the nearby formerly hostile Klingon empire finally conceded and joined their ranks.
Resistance is futile.
Denying co-operation is all right. They'll return and give the proposition to a future generation who will probably agree their terms. No advanced space travel techniques. They'd probably hold off providing vital medicines as also being too advanced. They'll get you in the end by waiting for a more belligerent political regime.