Factions with closely aligned goals may choose to form an x20pxalliance through diplomacy. Allied factions commit at least to defending one another from foreign attack, and may go to war together. The military alliance is the highest form of alliance short of confederation or vassalisation. It has several benefits:
Allies share their lines of sight from both units and settlements.
Allied armies will reinforce one another in battle automatically.
Allies can coordinate their attacks on their mutual enemies. This is particularly useful when projecting power into less habitable areas.