It's basically a series of novels (no, not manga, actual novels) that got converted to anime. It's not a direct 1:1 conversion; some changes were made, but nothing was cut out.
It's probably the single most realistic depiction of a war I have ever seen in any medium. It doesn't just focus on the two leading strategists in the conflict, but explores every aspect of both nations, from the politics in both nations, to the joe nobodies who die by the hundreds of thousands, to the daily lives of the various admirals on both sides. It doesn't show one side as marauding demons and the other as shining beacons of hope, but purely a land/political dispute between two massive nations. One of the main characters even says at one point "War is very rarely good versus evil, more often than not it's good vs good."
The characters are very real, very fleshed out, and you find yourself getting attached to pretty much all of them.
It is NOT an action series. The fleet battles are often very long and drawn out, though without being unending powerlevel fights, and the infantry combat isn't the focus of the series.
If you like a good drama series, you'll like Legend of Galactic Heroes