Advanced Strategy, which requires Tanks and Infantry to always stick together. Tanks get +1 HP and +1 attack, but -3 defence (bringing it down to 1). Infantry gets +1 HP and +1 defence, but -3 attack.
Your army uses heavier and less vulnerable units - however, they come with the increased cost and slower movement. +3 HP, +20 cost, -2 movement for all units.