When the technology to revive fallen soldiers was invented early during the war, it gave one army a huge advantage. Or at least, that's what they thought.
When a soldier died in combat, they were taken to a "respawn station", where they were revived and sent back out to fight.
The enemy army got ahold of the technology later in the war, by the time the soldiers who had been revived over and over again were starting to go insane.
Oh, yeah, that was the kicker. Being brought back to life ripped away pieces of your sanity, little by little. How could a soldier die over and over again, only to be sent back out into endless death and destruction?
The enemy army didn't exactly "win." Rather, both armies just went completely insane.