The Space Marines were pretty clever. With their advanced science, they simply beamed their dead off the battlefield to be recycled into another wave of soldiers. A gruesome and morally questionable mechanism of war, but one that led them to unstoppable success against any threat.
The enemy Xenomorphs would prove to be quite a different story. Wild, violent, but spectacularly intelligent, the alien creatures found they could simply nest inside their attacker's chests. This allowed them to follow the dead soldier as they're recycled into new, fresh men... behind enemy lines! Luckily, this cunning tactic was discovered early on.