An early screenshot released by Taleworlds of the Army UI
An Army is a new concept introduced in Mount&Blade II: Bannerlord as part of the singleplayer side of the game. Armies are a replacement for Campaigns as they work very similarly but are still fundamentally different.
Armies require Influence to maintain, and once the leader of the Army no longer has the necessary influence to keep the Army together, it will disband. However, influence can be gained while leading an Army, this means that an army could theoretically be maintained by the influence it gains and maintain itself indefinitely.
The largest difference between Armies and Campaigns, is that Armies are like their own Party. In Warband, Campaigns were a grouping of lords on the overland map, each following the leader. Armies however, are a single unit moving about the map with their own speed, morale and food supplies. This means that tactics familiar to many Warband players against Campaigns will no longer be usable against Armies.
Armies have their own unique user interface, containing important information about the Army's capacity to wage war. The UI shows the Army morale, speed and food, as well as the lords in the Army.
Official Description[]
Parties can be assembled together to form armies. Armies have an advantage over singular parties since they can hold a greater number of troops and display better coordination. The leader of an army will have to set it up first by spending Influence; but parties led by relatives or companions affliated with the player's clan cost no influence to to ask to join. Each army has an Army Cohesion stat which is crucial to its operation. An army will be disbanded automatically if its cohesion drops too low.