We were overrun before we knew it. Not an invasion, precisely. An infestation. Their ships stole when our backs were turned, nibbling away at our nascent web of interstellar travel. Rats. Space rats.
We sent cats to fight them. They hunted their prey with cunning and agility. The cats never shared a kill, but sometimes they'd leave a bit of wreckage in a docking bay.
When the last of the foes was gone, we waited for our heroes to return, but they didn't. We received one last communication from the Alpha Cat: "BRB. Chasing a comet."
They never came back.