I think the magnitude of the difference between the opposing parties is a lot greater when it comes to comparing humans to an alien civilization capable of projecting some force light years away from their home in a reasonable amount of time.
It's more like neolithic man meets nuclear submarine... but worse. We have 0 tools at our disposal to fight anything that's farther away than the edge of the atmosphere. "They" can take a few decades to accelerate a really massive "civilization ending" object and slam it into us. It's not like we can go anywhere to avoid it.
Why do you assume these UFOs reached the Earth in a "reasonable" amount of time? The possibility that they were roaming the space for hundreds or even thousands of years before reaching our planet is as reasonable as any other.
Even if an alien civilization finds our planet, I don't see why that would imply we are inside their logistic operational range.
A well equipped squad of marines would still eventually succumb to a gird of barbarians unless they could resupply.