> There's a Sci-fi story - I forget the name - where space travel is actually quite simple if you know the math to make it happen, and when a peaceful alien race visits Earth, the humans - with their thousands of years of armed conflict - kill the aliens and take their spaceships.

I think you might be thinking of Harry Turtledove's "The Road Not Taken". They weren't exactly friendly, but they were quite surprised by Man's ingenuity.