When the Achuan invaded, they apologised.
At its most basic level, what they said translated into English as, “Uh, we didn’t mean to nuke your planet with our war debris. Sorry.”
The vast swathes of three continents that had been peppered with ‘hot’ technology falling from the sky were not impressed with their apology or the invasion. Until the aliens said they could fix it.
There was a white powder that they sprinkled on everything that was overly radioactive – even things that weren’t the result of falling, shot up star ships. The radioactivity stopped.
Then the Achuan began trying to collect the broken pieces of starship. Trouble was, once the debris was no longer radioactive, the human Terrani swarmed all over it. Bits were gone, disappeared into workshops, laboratories and backyard sheds all over the planet. When they realised that the Achuan got upset, stamped their feet and yelled.
Someone human, in authority, observing this behaviour thought to ask, very cautiously, “Are you an adult?” Into the silence the human suggested, “Perhaps we should speak to your responsible adult?”
The Achuan produced a member of their species’ mentoring species.
“I’ve heard the Achuans’ explanation of the situation,” the newcomer said, its sounds being translated into Achuan and the particular Terrani language the human spoke by glowing orbs that circled its head, “and now I want to hear your view of it.” It turned to the suddenly protesting Achuan and told them, “Go and wait over there!” The Achuan crossed the valley and waited while the newcomer listened.
When the human finished the newcomer spoke again. “What you’ve told me matches what my systems tell me. These striplings think I’m a nice old man who’ll be easier on them than their parents. I’m about to disabuse them. They meant well but that white power is for small scale industrial use and should’ve been cleaned up straight away.” It sighed. “Rehabilitating this mess will take time. Would any of your people consider emigration to your own colony worlds in the meantime?”
And then a human demonstrated that you could reverse engineer a stardrive.