In the stand-alone novel I read 1-2 years ago on Amazon, a young soldier (White) in armor (full body and super expensive) fights aliens on a planet. I think it is his home planet and he is familiar with the rough terrain.
The humans got involved in the war to stop these aliens from exterminating another alien species. They sent thousands of probes into the aliens' space to try and see how the war was going but they were all destroyed. Everyone warned the humans to not get involved; the aliens they were provoking played for keeps. They killed everyone they ever fought.
The novel picks up after the war is lost and the main character's squad (4-5 troops) is all that is left. They are out of radio/update range of headquarters so they do not know how badly the war has gone. As the squad keeps losing soldiers to enemy attacks, eventually they get an update from command frequency which makes their sergeant (female) the commander-in-chief of the alliance/confederacy meaning they are the last human soldiers alive.
After the sergeant dies and makes the main character commander-in-chief over his corporal/love interest he gets an option to implement a fail-safe. He chooses not to, and he and his love interest are eventually captured. They are lead to a ritual to be killed because they are the last human warriors alive and the aliens want to honor them. They are lead down a huge gauntlet of alien warrior towards a stage, where right before they are to be killed the main character relents and sets off the fail-safe.
The fail-safe turns out to be having all the nukes the government hid under every planet they ever held in the war explode to kill as many aliens as possible. After the human government realized they would lose the war they started seeding planets (alien and human) with nukes to kill as many as possible once humanity was exterminated. This was to make sure the colony ships they had launched would be able to escape the aliens' attention as they would lose billions from the exploded planets. The novel ends with the main character and his lover dying along with billions of aliens as a convoy with the last humans escapes into space to start over.