The Federated Army is the unified military command which serves The Federated Territories. It encompasses both the ground forces and space fleets. They are a benevolent organization trying to identify and eliminate all threats to the territories, although were unaware of The Great Evil's nature initially.
The Federated Army received information from Father Vito Cornelius at the military headquarters regarding the Evil. At the same time, one of their warships was first seen flying towards The Great Evil around Jupiter's orbit, and after deploying a satellite which accidentally gave the Evil the ability to communicate, attempted to destroy it with its missiles. That failed, only causing the Evil to grow larger with each missile barrage. The captain proved to be a bit incompetent after seeing the results, and despite the President's order to retreat, suffered bleeding from the Evil's influence and the ship was destroyed by a flaming skull.
Three more warships were sent to track it's movements and behaviour, but not to attack, given the failure of conventional weapons in this scenario.
The Army instead hired Korben Dallas via General Munro to have him retrieve the stones that form part of an ancient weapon to defeat the Evil. He was sent to the Fhloston Paradise via a contest rigged by the Army, and they also allowed Dallas to take Leeloo with him. They did not know that Vito also smuggled himself inside the same starliner to Fhloston Paradise.
The Army High Command acted as mission control for the covert operation aboard the Fhloston Paradise that Dallas is taking part in, although they did not send additional military units. When the Mangalores attacked the ship and held everyone hostage, the Army High Command made sure that Dallas helped the ship's security forces regain control of the ship.
They possibly guided Dallas and company back to the temple at Earth to initiate the weapon once the stones were collected from the diva that died from Mangalore attack.
The Evil was killed, and the Army declared Dallas and Leeloo (the fifth element) the heroes of the Federated Territories for their efforts.