Whenever you or your allies are attacked, other alliance members can help by reinforcing the defender. Reinforcing allows you to send a specific amount of troops to another map tile or player base to help defend against an attack. To reinforce an ally you need to select the desired base and chose the Reinforce option
Every player has a different reinforcement capacity. If the troop amount that is sent to the defender exceeds his reinforce capacity, all excess troops will automatically return to their owners. The same will occur also if the capacity is exceeded after the conclusion of battle. Reinforce capacity can be increased by completing Civil District improvements.