Imagine a post-apocalyptic world in 2513. Unknown to us here and now, aliens first visited a remote part of the planet in 1992. The aliens were just exploring and didn't really have any nefarious plans, in fact they were on their way to go trade with some other aliens and kind of ended up here by accident. They poked around a bit in the small corner of the globe they landed on, took some samples, did a little surveying, said “hi” to the rather surprised locals and then traded some of their stuff for some of the locals' stuff.