Army & Federal response adds the Feds & the Army to the 4th and 5th star wanted level.

At first, the Feds will join the fight (4 stars) and will try to take you down.
An order from the DoD (Departement of Defense) came and all agencies are on you now. FIB and IAA agents are gonna team up to beat you.

At 5 stars, the Feds are called back after their failure to kill you. It's now time for the DoD to take control of the affairs. Insurgents, Crusaders, Tanks, Valkyries and Barracks full or trained army personal will chase you and take you down.

If you manage to kill enough of them, they will eventually give up and you will have succefully pushed all the forces of San Andreas.