Index: binaries/data/mods/public/gui/session/selection.js
===================================================================
--- binaries/data/mods/public/gui/session/selection.js
+++ binaries/data/mods/public/gui/session/selection.js
@@ -285,6 +285,7 @@
 		return;
 
 	let added = [];
+	ents = this.addBattalionMembers(ents);
 
 	for (const ent of ents)
 	{
@@ -326,7 +327,8 @@
 
 EntitySelection.prototype.removeList = function(ents)
 {
-	var removed = [];
+	let removed = [];
+	ents = this.addBattalionMembers(ents);
 
 	for (let ent of ents)
 		if (this.selected.has(ent))
@@ -410,6 +412,7 @@
 EntitySelection.prototype.setHighlightList = function(ents)
 {
 	const highlighted = new Set();
+	ents = this.addBattalionMembers(ents);
 	for (const ent of ents)
 		highlighted.add(ent);
 
@@ -459,7 +462,25 @@
 	this.addList([entityID]);
 
 	Engine.CameraMoveTo(entState.position.x, entState.position.z);
-}
+};
+
+/**
+ * Adds the battalion members of a selected entities to the selection.
+ * @param {number[]} entities - The entity IDs of selected entities.
+ * @return {number[]} - Some more entity IDs if part of a battalion was selected.
+ */
+EntitySelection.prototype.addBattalionMembers = function(entities)
+{
+	if (entities.length)
+	{
+		const battalions = Engine.GuiInterfaceCall("GetBattalions", { "players": undefined });
+		for (const battalion of battalions)
+			if (entities.some(entity => battalion.entities.indexOf(entity) != -1))
+				entities = entities.concat(battalion.entities.filter(entity => entities.indexOf(entity) == -1));
+	}
+
+	return entities;
+};
 
 /**
  * Cache some quantities which depends only on selection
Index: binaries/data/mods/public/gui/session/selection_panels_helpers.js
===================================================================
--- binaries/data/mods/public/gui/session/selection_panels_helpers.js
+++ binaries/data/mods/public/gui/session/selection_panels_helpers.js
@@ -318,6 +318,34 @@
 	});
 }
 
+/**
+ * @param {number[]} entities - The entity IDs of the entities.
+ */
+function formBattalion(entities)
+{
+	if (!entities || !entities.length)
+		return;
+
+	Engine.PostNetworkCommand({
+		"type": "formbattalion",
+		"entities": entities
+	});
+}
+
+/**
+ * @param {number[]} battalions - The IDs of the battalions to disband.
+ */
+function disbandBattalion(battalions)
+{
+	if (!battalions || !battalions.length)
+		return;
+
+	Engine.PostNetworkCommand({
+		"type": "disbandbattalion",
+		"battalions": battalions
+	});
+}
+
 function performStance(entities, stanceName)
 {
 	if (!entities)
Index: binaries/data/mods/public/gui/session/unit_actions.js
===================================================================
--- binaries/data/mods/public/gui/session/unit_actions.js
+++ binaries/data/mods/public/gui/session/unit_actions.js
@@ -1491,6 +1491,52 @@
 		"allowedPlayers": ["Player"]
 	},
 
+	"formBattalion": {
+		"getInfo": function(entStates)
+		{
+			const refTemplateName = entStates[0].identity.selectionGroupName || entStates[0].template;
+			if (entStates.length < 2 ||
+				entStates.some(entState =>
+					(entState.identity.selectionGroupName || entState.template) != refTemplateName
+				))
+				return false;
+
+			return {
+				"tooltip": colorizeHotkey("%(hotkey)s" + " ", "session.formbattalion") +
+				           translate("Form a battalion with the currently selected units."),
+				"icon": "training.png",
+				"enabled": true
+			};
+		},
+		"execute": function(entStates)
+		{
+			if (entStates.length)
+				formBattalion(entStates.map(entState => entState.id));
+		},
+		"allowedPlayers": ["Player"]
+	},
+
+	"disbandBattalion": {
+		"getInfo": function(entStates)
+		{
+			if (entStates.every(entState => entState.battalion == false))
+				return false;
+
+			return {
+				"tooltip": colorizeHotkey("%(hotkey)s" + " ", "session.disbandbattalion") +
+				           translate("Disband all currently selected battalion(s)."),
+				"icon": "cancel.png",
+				"enabled": true
+			};
+		},
+		"execute": function(entStates)
+		{
+			if (entStates.length)
+				disbandBattalion(entStates.map(entState => entState.battalion));
+		},
+		"allowedPlayers": ["Player"]
+	},
+
 	"garrison": {
 		"getInfo": function(entStates)
 		{
Index: binaries/data/mods/public/simulation/components/BattalionManager.js
===================================================================
--- /dev/null
+++ binaries/data/mods/public/simulation/components/BattalionManager.js
@@ -0,0 +1,181 @@
+function BattalionManager() {}
+
+BattalionManager.prototype.Schema =
+	"<a:component type='system'/><empty/>";
+
+BattalionManager.prototype.Init = function()
+{
+	this.battalions = new Map();
+	this.nextId = 0;
+};
+
+/**
+ * Creates a new battalion.
+ * This now reuses IDs of disbanded battalions.
+ * @param {number[]} entities - The entity IDs to form the battalion with.
+ */
+BattalionManager.prototype.FormBattalion = function(player, entities)
+{
+	this.battalions.set(this.nextId++, {
+		"player": player,
+		"entities": new Set()
+	});
+	this.AddEntities(id, entities);
+};
+
+/**
+ * Removes battalion(s).
+ * @param {number[]} battalionIDs - The IDs of the battalions. If undefinded all battalions will be removed.
+ */
+BattalionManager.prototype.DisbandBattalions = function(battalionIDs = undefined)
+{
+	if (battalionIDs)
+		for (const id of battalionIDs)
+			this.battalions.delete(id);
+	else
+		this.battalions.clear();
+};
+
+/**
+ * Adds an entity/entities to the battalion.
+ * @param {number} battalionID - The unique ID of the battalion.
+ * @param {number} battalionID - The index of the battalion, passed by "formBattalion".
+ * @param {number[]} entities - The entity IDs to add to the battalion.
+ */
+BattalionManager.prototype.AddEntities = function(battalionID, entities)
+{
+	let battalion = this.battalions.get(battalionID);
+	if (!battalion)
+	{
+		error("Entity/Entities " + entities + " could not be added to battalion " + battalionID + " for that battalion does not exist.");
+		return;
+	}
+	const cmpGuiInterface = Engine.QueryInterface(SYSTEM_ENTITY, IID_GuiInterface)
+	for (const entity of entities)
+	{
+		// Entities cannot be in more than one battalion at the same time.
+		let bat = this.PartOfBattalion(entity);
+		if (bat !== -1)
+			this.RemoveEntities(bat, [entity]);
+
+		const entState = cmpGuiInterface.GetEntityState(battalion.player, entity);
+
+		// When this function is called during group rebuild, deleted
+		// entities will not yet have been removed, so entities might
+		// still be present in the group despite not existing.
+		if (!entState)
+			continue;
+
+		battalion.entities.add(entity);
+		if (!battalion.template)
+			battalion.template = entState.selectionGroupName || entState.template;
+	}
+};
+
+/**
+ * Removes an entity/entities from the battalion.
+ * @param {number} battalionID - The unique ID of the battalion.
+ * @param {number} entities - The entity ID of the entities to remove.
+ */
+BattalionManager.prototype.RemoveEntities = function(battalionID, entities)
+{
+	const battalion = this.battalions.get(battalionID);
+	for (const entity of entities)
+		battalion.entities.delete(entity);
+};
+
+/**
+ * Check wheter an entity is part of an battalion.
+ * @param {number} entity - The entity ID to check.
+ * @return {number} - Either a number when the entity is part of a battalion
+ *				or "0" when it is not.
+ */
+BattalionManager.prototype.PartOfBattalion = function(entity)
+{
+	for (const [id, battalion] of this.battalions.entries())
+		if (battalion.entities.has(entity))
+			return id;
+
+	return 0;
+};
+
+/**
+ * Update the battalions. E.g. when units die or change ownership and such.
+ * ToDo: Run this when necessary.
+ */
+BattalionManager.prototype.Update = function()
+{
+	this.CheckRenamedEntities();
+	const battalionsToDisband = [];
+	for (const [id, battalion] of this.battalions.entries())
+	{
+		const entitiesToRemove = [];
+		for (const entity of battalion.entities)
+		{
+			const entState = Engine.QueryInterface(SYSTEM_ENTITY, IID_GuiInterface).GetEntityState(battalion.player, entity);
+			// Remove deleted units or now unowned units.
+			if (!entState || entState.player != battalion.player)
+				entitiesToRemove.push(entity);
+		}
+		this.RemoveEntities(id, entitiesToRemove);
+
+		if (!battalion.entities.length)
+			battalionsToDisband.push(id);
+	}
+	if (battalionsToDisband.length)
+		this.DisbandBattalions(battalionsToDisband);
+};
+
+/**
+ * Update battalions if some entities in the battalion were renamed
+ * (in case of unit promotion or finishing building structure).
+ * ToDo: This needs some love.
+ */
+BattalionManager.prototype.CheckRenamedEntities = function()
+{
+	const cmpGuiInterface = Engine.QueryInterface(SYSTEM_ENTITY, IID_GuiInterface);
+	// Reconstruct groups if at least one entity has been renamed.
+	const renamedEntities = cmpGuiInterface.GetRenamedEntities();
+	if (renamedEntities.length == 0)
+		return;
+
+	for (const [id, battalion] of this.battalions.entries())
+	{
+		renamedEntities = cmpGuiInterface.GetRenamedEntities(battalion.player);
+		for (const renamedEntity of renamedEntities)
+		{
+			if (battalion.entities.has(renamedEntity.entity))
+			{
+				this.RemoveEntities(id, [renamedEntity.entity]);
+				const cmpOwner = Engine.QueryInterface(renamedEntity.newentity, IID_Ownership);
+				if (cmpOwner && cmpOwner.GetOwner() == battalion.player)
+					this.AddEntities(id, [renamedEntity.newentity]);
+			}
+		}
+	}
+};
+
+/**
+ * Return the battalion(s).
+ * @param {number[]} players - Optionally the players we want to check.
+ * @return {object[]} battalions - The currently used battalions.
+ */
+BattalionManager.prototype.GetBattalions = function(players = undefined)
+{
+	if (!players)
+		return this.battalions;
+
+	return this.battalions.filter(battalion => players.indexOf(battalion.player) != -1);
+};
+
+
+/**
+ * Subscribe to all changes in ownership and such.
+ */
+BattalionManager.prototype.OnGlobalOwnershipChanged = function(msg)
+{
+	if (this.PartOfBattalion(msg.entity) !== -1)
+		this.Update();
+};
+
+Engine.RegisterSystemComponentType(IID_BattalionManager, "BattalionManager", BattalionManager);
Index: binaries/data/mods/public/simulation/components/GuiInterface.js
===================================================================
--- binaries/data/mods/public/simulation/components/GuiInterface.js
+++ binaries/data/mods/public/simulation/components/GuiInterface.js
@@ -301,6 +301,9 @@
 		ret.maxCapturePoints = cmpCapturable.GetMaxCapturePoints();
 	}
 
+	let cmpBattalionManager = Engine.QueryInterface(SYSTEM_ENTITY, IID_BattalionManager);
+	ret.battalion = cmpBattalionManager.PartOfBattalion(ent);
+
 	let cmpBuilder = Engine.QueryInterface(ent, IID_Builder);
 	if (cmpBuilder)
 		ret.builder = true;
@@ -1036,6 +1039,11 @@
 	return Engine.QueryInterface(SYSTEM_ENTITY, IID_RangeManager).GetNonGaiaEntities();
 };
 
+GuiInterface.prototype.GetBattalions = function(player, data)
+{
+	return Engine.QueryInterface(SYSTEM_ENTITY, IID_BattalionManager).GetBattalions(data.players);
+};
+
 /**
  * Displays the rally points of a given list of entities (carried in cmd.entities).
  *
@@ -2097,6 +2105,7 @@
 	"IsFormationSelected": 1,
 	"GetFormationInfoFromTemplate": 1,
 	"IsStanceSelected": 1,
+	"GetBattalions": 1,
 
 	"UpdateDisplayedPlayerColors": 1,
 	"SetSelectionHighlight": 1,
Index: binaries/data/mods/public/simulation/components/interfaces/BattalionManager.js
===================================================================
--- /dev/null
+++ binaries/data/mods/public/simulation/components/interfaces/BattalionManager.js
@@ -0,0 +1 @@
+Engine.RegisterInterface("BattalionManager");
Index: binaries/data/mods/public/simulation/components/tests/test_BattalionManager.js
===================================================================
--- /dev/null
+++ binaries/data/mods/public/simulation/components/tests/test_BattalionManager.js
@@ -0,0 +1,74 @@
+Engine.LoadComponentScript("interfaces/BattalionManager.js");
+Engine.LoadComponentScript("BattalionManager.js");
+
+let cmpBattalionManager = ConstructComponent(SYSTEM_ENTITY, "BattalionManager");
+
+let player = 1;
+let group1 = [2, 3, 4, 5];
+let group2 = [6, 7, 8, 9];
+let group3 = [5, 6, 10, 11];
+let allEnts = [2, 3, 4, 5, 6, 7, 8, 9, 10, 11];
+let template = "{civ}/{faction}/{entity}/{rank}";
+let GUItemplate = {
+	"template": template,
+	"player": player
+};
+
+AddMock(SYSTEM_ENTITY, IID_GuiInterface, {
+	"GetEntityState": (owner, entity) => GUItemplate,
+	"GetRenamedEntities": () => []
+});
+
+// No entity is in a battalion yet.
+for (let i of allEnts)
+	TS_ASSERT_EQUALS(cmpBattalionManager.PartOfBattalion(i), 0);
+TS_ASSERT_UNEVAL_EQUALS(cmpBattalionManager.GetBattalions(), []);
+
+cmpBattalionManager.FormBattalion(player, group1);
+for (let i of group1)
+	TS_ASSERT_EQUALS(cmpBattalionManager.PartOfBattalion(i), 1);
+
+cmpBattalionManager.FormBattalion(player, group2);
+for (let i of group2)
+	TS_ASSERT_EQUALS(cmpBattalionManager.PartOfBattalion(i), 2);
+
+// No battalions for player 2.
+TS_ASSERT_UNEVAL_EQUALS(cmpBattalionManager.GetBattalions([2]), []);
+
+// Here we form a battalion with parts of another battalion, in-game this should only
+// happen when merging complete battalions, since they are only fully selectable.
+cmpBattalionManager.FormBattalion(player, group3);
+for (let i of group3)
+	TS_ASSERT_EQUALS(cmpBattalionManager.PartOfBattalion(i), 3);
+for (let i of group1)
+	if (i == 5)
+		TS_ASSERT_EQUALS(cmpBattalionManager.PartOfBattalion(i), 3);
+	else
+		TS_ASSERT_EQUALS(cmpBattalionManager.PartOfBattalion(i), 1);
+for (let i of group2)
+	if (i == 6)
+		TS_ASSERT_EQUALS(cmpBattalionManager.PartOfBattalion(i), 3);
+	else
+		TS_ASSERT_EQUALS(cmpBattalionManager.PartOfBattalion(i), 2);
+
+cmpBattalionManager.DisbandBattalions([1]);
+let result = [
+	{ "id":2, "player": player, "entities": [7, 8, 9], "template": template },
+	{ "id":3, "player": player, "entities": [5, 6, 10, 11], "template": template }
+];
+TS_ASSERT_UNEVAL_EQUALS(cmpBattalionManager.GetBattalions(), result);
+
+// Verify that a removed entity is indeed not part of a battalion anymore.
+cmpBattalionManager.RemoveEntities(2, [8]);
+TS_ASSERT_EQUALS(cmpBattalionManager.PartOfBattalion(8), 0);
+
+// Verify that once the last entity of a battalion is removed the battalion is disbanded.
+cmpBattalionManager.RemoveEntities(3, [5, 6, 10, 11]);
+cmpBattalionManager.Update();
+result = [{ "id":2, "player": player, "entities": [7, 9], "template": template }];
+TS_ASSERT_UNEVAL_EQUALS(cmpBattalionManager.GetBattalions(), result);
+
+cmpBattalionManager.FormBattalion(player, allEnts);
+cmpBattalionManager.Update();
+result = [{ "id":1, "player": player, "entities": allEnts, "template": template }];
+TS_ASSERT_UNEVAL_EQUALS(cmpBattalionManager.GetBattalions(), result);
Index: binaries/data/mods/public/simulation/components/tests/test_GuiInterface.js
===================================================================
--- binaries/data/mods/public/simulation/components/tests/test_GuiInterface.js
+++ binaries/data/mods/public/simulation/components/tests/test_GuiInterface.js
@@ -41,6 +41,7 @@
 Engine.LoadComponentScript("interfaces/Upgrade.js");
 Engine.LoadComponentScript("interfaces/Upkeep.js");
 Engine.LoadComponentScript("interfaces/BuildingAI.js");
+Engine.LoadComponentScript("interfaces/BattalionManager.js");
 Engine.LoadComponentScript("GuiInterface.js");
 
 Resources = {
@@ -585,6 +586,10 @@
 	"GetRates": () => ({ "food": 2, "wood": 3, "stone": 5, "metal": 9 })
 });
 
+AddMock(SYSTEM_ENTITY, IID_BattalionManager, {
+	"PartOfBattalion": () => false
+});
+
 // Note: property order matters when using TS_ASSERT_UNEVAL_EQUALS,
 //	because uneval preserves property order. So make sure this object
 //	matches the ordering in GuiInterface.
@@ -606,6 +611,7 @@
 	"maxHitpoints": 60,
 	"needsRepair": false,
 	"needsHeal": true,
+	"battalion": false,
 	"builder": true,
 	"visibility": "visible",
 	"isBarterMarket": true,
Index: binaries/data/mods/public/simulation/helpers/Commands.js
===================================================================
--- binaries/data/mods/public/simulation/helpers/Commands.js
+++ binaries/data/mods/public/simulation/helpers/Commands.js
@@ -680,6 +680,20 @@
 		}
 	},
 
+	"formbattalion": function(player, cmd, data)
+	{
+		const cmpBattalionManager = Engine.QueryInterface(SYSTEM_ENTITY, IID_BattalionManager);
+		cmpBattalionManager.FormBattalion(player, data.entities);
+		cmpBattalionManager.Update();
+	},
+
+	"disbandbattalion": function(player, cmd, data)
+	{
+		const cmpBattalionManager = Engine.QueryInterface(SYSTEM_ENTITY, IID_BattalionManager);
+		cmpBattalionManager.DisbandBattalions(cmd.battalions)
+		cmpBattalionManager.Update();
+	},
+
 	"lock-gate": function(player, cmd, data)
 	{
 		for (let ent of data.entities)