Forever series (Joe W. Haldeman)

The Earth's leaders have drawn a line in the interstellar sand, despite the fact that the fierce alien enemy they would oppose is inscrutable, unconquerable, and very far away. Humans first bumped heads with the Taurans when we began using collapsars to travel the stars. Although the collapsars provide nearly instantaneous travel across vast distances, the relativistic speeds associated with the process means that time passes slower for those aboard...

"The forever war raged for centuries ... but Marygay was there to see it all. During the interstellar travel that took her years, generations came and went on Earth. Thanks to special relativity, Marygay was able to wait for many lifetimes to be reunited with her lost love, William Mandella. But they, and the other survivors of the war, are struggling to find their place in the universe after the galaxy shattering conflict"--