Combat research unlocks new troops as well as increasing their effectiveness. The combat research tree also includes bonuses to scouting and troop training speed. Broadly speaking troops can be broken down into four tiers with a significant increase in attack and defence for each tier.
Strategic Combat research allows you to unlock all of the Strategic Troops and increase their attack, defense, and health. Strategic Troops follow the same four tiers as standard troops, except they are each stronger against one specific troop type and weak against two troop types.