I see that's a Harry Turtledove novel. He also wrote the World War series in which basically an advanced reptile race invades Earth with nukes and hydrogen fuelled tanks and shit during the height of WWII. The Allies and Axis have to unite to defeat them. I think he then wrote a followup story where they went out into space to push the alien shit in even further. The Aliens had sent a probe to Earth some 1000 years earlier and just seen knights in shining armour and shit, and as they and all the alien races they knew evolved socially and technologically at a snail's pace, they expected little more and were shocked at the speed with which we adapted.
It also reminds me of a short story by Asimov that I can't remember the name of any more. It's one of the few Asimov stories with aliens in it, and is basically an interstellar version of the Greco-Persian wars, where there is a huge galactic alien empire and a much smaller group of constantly warring human planets. When the aliens attack the smaller humans, the humans unite and go medieval on the aliens. Good stuff.