The Forever War by Joe HaldemanRead about the other stories on the list.
In this tragic war story, humans believe that we've been attacked by aliens, so we send out ships to destroy the Tauran ships, and massacre their people. We don't bother to try and communicate with the Taurans for years, until we finally develop our own clones who can talk to the Taurans — and then we find out that we were the aggressor in the war.
Something similar plays out in Ender's Game and the Orphan's War series by Robert Buettner.
The Forever War made Charlie Jane Anders's list of the 12 greatest science fiction war stories.
--Marshal Zeringue