Who beats whom
Every unit has something it is good against, and something it should fear.
On land, Assault Squads beat infantry, Missile Squads beat tanks and aircraft, and tanks beat vehicles and buildings. Anti-Aircraft Systems guard the army from above.
At sea, submarines hunt carriers and landing ships, destroyers hunt submarines, and Attack Jets strike ships on the surface.
In the air, fighters beat other aircraft, Attack Jets hit ground forces and bombers smash buildings.
Some pairs simply cannot fight each other: fighters cannot hit tanks, and tanks cannot hit jets. When nobody can hit anybody, there is no battle.