In Joe Haldeman’s novel The Forever War, the alien species humans fight is the Taurans.
The novel begins with physics student William Mandella being conscripted into the United Nations Exploratory Force. Humanity is at war with the Taurans, who were first encountered after apparently attacking human colonists’ ships. The conflict sends Mandella and other soldiers across interstellar distances using collapsars, allowing them to traverse enormous distances almost instantaneously from a travel perspective.
The war’s defining complication is relativistic time dilation. Although missions may feel relatively short to the soldiers, centuries can pass on Earth. As Mandella returns from successive deployments, human society, technology, language, and military policy change dramatically. The alien war therefore becomes a framework for examining how separation and time can make soldiers strangers in their own civilization.
The Taurans are not the Ssi-ruuk, Thrint, or Kzinti, which belong to other science-fiction settings. Another important detail is that the Taurans are initially portrayed through human assumptions about aggression; the novel later complicates the moral clarity of the conflict and questions whether the war was understood correctly.