I am looking for a sci-fi book from the '80s or '90s. On the blue cover there was an astronaut, bearded and balding I think. The story was about humanity who fought a war with an alien race and won, barely. They won because they figured out teleportation, and a teacher who became a commander led the resistance to victory.
The protagonist of the book finds out some secrets of the conflict after the war. The alien ships are often described as having a round or circular shape. One story I remember of the conflict was that humanity rigged an explosion to trigger a sun to go supernova to take out the alien fleet but it didn't come to fruition because a woman took out the man who disabled the automatic trigger and wanted to trigger the explosion manually. Another story was the protagonist had an affair with a woman who had a crash on an ocean planet with her shuttle and drowned.
Humanity now lives on several planets. Not exactly sure how many. The book didn't seem to be part of a series, but rather a standalone novel. Any ideas?