Pre-God of War
Kratos' Army was with Kratos on the day of the immense battle with the Barbarians. As shown throughout the series, it was here that Kratos was indebted to Ares, God of War. They fought valiantly, but were easily overpowered by the ruthless tribes of Barbarians until Ares gave Kratos the ability to slaughter his enemies. It's also safe to assume that Kratos' former Spartan Soldiers were also the soldiers who followed him on his reign of terror during his servitude to Ares.
After Kratos became the new God of War, his soldiers removed any image that pertained to Ares and replaced them with that of Kratos. Kratos also found many bodies of Spartan Soldiers at the Mounts of Aroania, who were killed by the cold and dead hands of Erinys. Before Kratos left the city of Sparta, the Last Spartan gave Kratos his old weapons; a spear and a shield, an act which Kratos favored greatly.
Kratos also commanded a huge force of Spartans, which ravaged and conquered many cities in Greece in his name. When the Ghost of Sparta fought his battle against the Colossus of Rhodes, his soldiers lain siege to the city. When Zeus descended from Mount Olympus, he slaughtered Kratos' Soldiers, and only one lived to return to Sparta. Unfortunately for Kratos, on his journey to change his fate, he mortally wounded and killed the Last Spartan, which left Kratos the last remnant of the great army and Sparta. However, due to Kratos interfering the past, Kratos' Army was likely resurrected along with the Last Spartan. Unfortunately, they probably perished again after Poseidon's death sent a great flood upon the world.