Hawks and other shorter ranged units
OK, so read the line about hawks protecting long range units. This is not the case. Putting hawks or any other short range unit essentially puts them out in front of long range units and since they are fast, they engage FIRST and get totally wiped out. Your long range units are farther away and the attacking army will have shot at them after regeneration (a couple seconds) This is especially true for defense grid bases. In bases without a defense grid (sometimes called mech bases) the hawks will destroy incoming attacks, but the incoming units have priority targets - and the first priority is always long range units. The hawks under most circumstances will destroy the incoming long range attacks but they shoot simultaneously, so the incoming stuff does get hit and destroyed, their shot will priority target helicopters and artillery (long range unit)
At a mech base put very small amounts of long range units, essentially to draw the attack, while the hawks and other short range kill incoming units. By small number I mean 50 each helicopter and hailstorm and 200 Artillery. Put larger numbers and they will get destroyed for no reason, essentially they serve as bait for incoming attacks.
Mech (short range) attacks are the most interesting because a multitude of units are involved and each unit has it's priority target. Hailstorms, for instance, attack artillery first, etc... Set up an attack with a friend, then watch the battle video closely, see which units shoot at which units in defense, then switch roles and watch the attack, from the other perspective.
But for the sake of my sanity, do not expect hawks to "protect" long range units - all they can do is hit them AFTER they hit the long range. So hawks survive and long range dont.