The Aliens by Murray Leinster audiobook.
The human race was expanding through the galaxy ... and so, they knew, were the Aliens. Who were these beings? Traces of them could be found scattered on planets everywhere, some very recent, but the aliens themselves were never encountered. They were obviously just as advanced technologically as humans and obviously looking for planets to expand to, just like humans. But what would happen when the two races, human and alien met? From history it was obvious that a war should be planned for, two expanding empires cannot tolerate rivals and they would clash and it could happen at any time. Would it be a war to the death? Sadly, that was the most probable outcome. Hundreds of human ships were designed specifically to frantically comb the known universe to gather information about them to prepare for war. Which was inevitable of course.