The Alien Army was bred by USCMC General Thomas Spears to serve as his elite fighting force, trained and selected for a mission to reclaim Earth from the Aliens during the events of the Earth War. The army itself entirely consisted of Drones and a single Queen that Spears would use to keep them in line.
After the initial Alien infestation on Earth, General Spears started his own secret project to harness the Aliens and control their behavior to create his own Alien Army to reclaim Earth. When the war broke out at CL5, Spears set up station off-world, outside a local Hive and ran his army into the underground, experimenting on the Aliens trying to control their mannerisms. Spears used a nearby colony as a breeding ground for him to acquire more Alien specimens. Throughout his experiments, Spears used a Queen as the means to control the Aliens by threatening to incinerate her offspring every time they disobey his orders. The Aliens began to slowly obey Spears' commands, thinking that this was progress in his experiment to gain the Aliens' obedience. In reality, this was merely an act to fool Spears into thinking that he had actual control over them. The Drones Spears selected are each branded with a numbers on their forehead, they are then stored within stasis tubes aboard his ship, the McArthur until their deployment on Earth.
Just as Hicks, Newt and Butler arrived at Spears' base, several of Spears' own subordinates, headed by Lieutenant Powell planned a coup to overthrow the General due to the nature of his experiments and his progressive obsession with the Aliens. Hicks and Newt joined the coup and together planned to ambush Spears once he returned. The rebels had Spears' loyalists imprisoned, but chaos ensued between the group and caused the captive Aliens to escape. Spears arrived and had the remaining rebels imprisoned while he prepares for departure. Spears launched the McArthur and left his men at the mercy of the Aliens, Newt and Hicks were able to board the ship on time before the launch. As the McArthur descend towards Earth, Spears ignored warnings from the Gateway Station and continued his mission, while Newt and Hicks escaped via escape pod. Shortly after arriving on Earth, Spears unleashed his army to reclaim Earth. The Aliens immediately betrayed Spears, much to the General's shock. Spears fended off the Aliens until he is confronted by the Queen, who had snuck aboard the McArthur. Angry at the Queen for fooling him, Spears fought the Queen, which resulted in both killing him and the Queen, ending his revolution before it even started. The remaining Aliens presumably disbanded the group.