No one holds the Realm alone. Band together with lords you trust: allies cannot raid, occupy or scout one another, and an ally's pass is as free to cross as your own. The map is colored by alliance, so allied lands read as one, with the alliance tag before each name (you can switch back to coloring by lord in the alliance panel).
Allies share what they learn: talk to them in the alliance channel at the bottom left; intel scouted by any ally is seen by the whole alliance (the channel announces who was scouted, and the raid view's odds use it); allied marches and enemies marching on an ally show on your map, and you are warned when an ally comes under attack.
You can station troops in an ally's city: when raiders come, the host's army and every garrison fight as one, behind the host's walls and under the host's defending hero; losses are shared by how many troops each brought, and renown by the strength each lent. A city houses 50 garrison troops per town hall level; garrisons take no hero and still eat their owner's food, the owner can withdraw them at any time, the host can send them home, and they all go home if the two of you stop being allies.
Allies can also strike a city together: call a rally, pick an enemy capital and a strike time (1, 2, 4, 8, 12, 24, 36 hours ahead), and your allies send troops to wait beneath its walls. At the strike time everyone there fights as one army, under the leader's general and bonuses; losses are shared by how many troops each brought, loot by what each side's survivors can carry, and renown by the strength each lent, and then everyone marches home. Troops that join bring no general and must reach the walls before the strike; the leader can call it off and anyone who joined can withdraw.
Founding an alliance costs 500 Gold; the founder becomes its leader. Others apply in the alliance panel and the leader or an officer accepts them — or the alliance can be open to all. An alliance holds at most 10 members and 2 officers.
The leader cannot simply leave: hand over leadership or disband first. After leaving or being expelled you must wait 12 hours before joining or founding another alliance.