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
@@ -289,6 +289,7 @@
 
 	let i = 1;
 	let added = [];
+	ents = this.addBattalionMembers(ents);
 
 	for (let ent of ents)
 	{
@@ -332,6 +333,7 @@
 EntitySelection.prototype.removeList = function(ents)
 {
 	var removed = [];
+	ents = this.addBattalionMembers(ents);
 
 	for (let ent of ents)
 		if (this.selected[ent])
@@ -388,6 +390,7 @@
 EntitySelection.prototype.setHighlightList = function(ents)
 {
 	var highlighted = {};
+	ents = this.addBattalionMembers(ents);
 	for (let ent of ents)
 		highlighted[ent] = ent;
 
@@ -429,6 +432,20 @@
 };
 
 /**
+ * Adds the battalion members of a selected entity to the selection.
+ * @param {number[]} ents - The entity IDs of selected entities.
+ * @return {number[]} - Some more entity IDs if part of a battalion was selected.
+ */
+EntitySelection.prototype.addBattalionMembers = function(ents)
+{
+	for (let battalion of g_Battalions.battalions)
+		if (ents.some(ent => battalion.ents.indexOf(ent) != -1))
+			ents = ents.concat(battalion.ents.filter(ent => ents.indexOf(ent) == -1));
+
+	return ents;
+};
+
+/**
  * Cache some quantities which depends only on selection
  */
 
@@ -505,3 +522,182 @@
 };
 
 var g_Groups = new EntityGroupsContainer();
+var g_Battalions = new BattalionsContainer();
+
+/**
+ * EntityBattalions class for managing entities in battalions (i.e. select one, select all).
+ */
+function BattalionsContainer()
+{
+	this.battalions = [];
+}
+
+/**
+ * Adds an entity/entities to the battalion.
+ * @param {number[]} ents - The creatures from LOTR.
+ */
+BattalionsContainer.prototype.formBattalion = function(ents)
+{
+warn("Selection form-function called! " + uneval(ents));
+	// Check whether there are free battalion spots.
+	for (let id = 0; true; id++)
+	{
+		if (!this.battalions.some(battalion => battalion.id == id))
+		{
+warn(id);
+			let index = this.battalions.length;
+warn(index);
+			this.battalions[index] = {"id": id};
+			this.addEntities(id, ents);
+			break;
+		}
+	}
+warn("Selection form-function finished! " + uneval(this.battalions));
+};
+
+/**
+ * Removes battalion(s).
+ * @param {number[]} battalionIDs - The unique IDs of the battalions.
+ */
+BattalionsContainer.prototype.disbandBattalion = function(battalionIDs)
+{
+warn("Selection disband-function called! " + uneval(battalionIDs));
+warn("Selection disband-function battalions: " + uneval(this.battalions));
+
+	this.battalions = this.battalions.filter(battalion => battalionIDs.indexOf(battalion.id) == -1);
+
+warn("Selection disband-function finished! " + uneval(this.battalions));
+};
+
+/**
+ * Adds an entity/entities to the battalion.
+ * @param {number} battalionID - The unique ID of the battalion, passed by "formBattalion".
+ * @param {number[]} ents - The creatures from LOTR.
+ */
+BattalionsContainer.prototype.addEntities = function(battalionID, ents)
+{
+warn("Selection add-function called! " + uneval(battalionID) + ": " + uneval(ents));
+	let index = +this.battalions.length - 1;
+	this.battalions[index].ents = [];
+	for (let ent of ents)
+	{
+		// Entities cannot be in more than one battalion at the same time.
+		let bat = this.isPartOfBattalion(ent);
+		if (bat !== false)
+			this.removeEntity(bat, ent);
+
+		if (this.battalions[index].ents.indexOf(ent) != -1)
+			continue;
+		let entState = GetEntityState(ent);
+
+		// 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;
+
+		let templateName = entState.template;
+		let key = GetTemplateData(templateName).selectionGroupName || templateName;
+
+		this.battalions[index].ents.push(ent);
+		if (!this.battalions[index].template)
+			this.battalions[index].template = key;
+	}
+};
+
+/**
+ * Removes an entity/entities from the battalion.
+ * @param {number} battalionID - The unique ID of the battalion.
+ * @param {number} ent - The entity ID of the entity to remove.
+ */
+BattalionsContainer.prototype.removeEntity = function(battalionID, ent)
+{
+warn("Selection remove-function called! " + uneval(battalionID) + ": " + uneval(ent));
+	for (let battalion of this.battalions)
+	{
+		if (battalion.id != battalionID)
+			continue;
+		let entIndex = battalion.ents.indexOf(ent);
+		if (entIndex != -1)
+		{
+			let battalionIndex = this.battalions.indexOf(battalion);
+			this.battalions[battalionIndex].ents.splice(entIndex, 1);
+		}
+	}
+warn("Selection remove-function finished! " + uneval(this.battalions));
+};
+
+/**
+ * Check wheter an entity is part of an battalion.
+ * @param {number} ent - The entity IDs to check.
+ * @return {number|boolean} - Either a number when the entity is part of a battalion
+ *				or false when it is not.
+ */
+BattalionsContainer.prototype.isPartOfBattalion = function(ent)
+{
+//warn("Selection isPartOfBattalion-function called! " + uneval(ent));
+//warn("Selection isPartOfBattalion-function battalions: " + uneval(this.battalions));
+
+	for (let battalion of this.battalions)
+		if (battalion.ents.indexOf(ent) != -1)
+			return battalion.id;
+
+//warn("Selection isPartOfBattalion-function? Nope!");
+	return false;
+};
+
+/**
+ * Update the battalions. E.g when units die or change ownership and such.
+ */
+BattalionsContainer.prototype.update = function()
+{
+warn("Selection update-function called! " + uneval(this.battalions));
+	this.checkRenamedEntities();
+	let battalionsToDisband = [];
+	let battalions = this.battalions;
+	for (let battalion of battalions)
+	{
+warn("Selection update-function iteration: " + uneval(battalions));
+		for (let ent of battalion.ents)
+		{
+			let entState = GetEntityState(+ent);
+			// Remove deleted units
+			if (!entState)
+				this.removeEntity(battalion.id, ent);
+		}
+warn("Selection update-function iteration: " + uneval(battalion));
+		if (!battalion.ents.length)
+			battalionsToDisband.push(battalion.id);
+	}
+	if (battalionsToDisband.length)
+		this.disbandBattalion(battalionsToDisband);
+warn("Selection update-function finished! " + uneval(this.battalions));
+
+};
+
+/**
+ * Update battalions if some entities in the battalion were renamed
+ * (in case of unit promotion or finishing building structure).
+ */
+BattalionsContainer.prototype.checkRenamedEntities = function()
+{
+	let renamedEntities = Engine.GuiInterfaceCall("GetRenamedEntities");
+	if (renamedEntities.length > 0)
+	{
+		let renamedLookup = {};
+		for (let renamedEntity of renamedEntities)
+			renamedLookup[renamedEntity.entity] = renamedEntity.newentity;
+
+		for (let battalion of this.battalions)
+			for (let renamedEntity of renamedEntities)
+				// Reconstruct the group if at least one entity has been renamed.
+				if (renamedEntity.entity in battalion.ents)
+				{
+					let oldEnts = battalion.ents;
+					// We probably want to keep the old battalion ID? Just "add entities" might do the job.
+					this.disbandBattalion(battalion.id);
+					this.formBattalion(oldEnts);
+					break;
+				}
+	}
+};
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
@@ -278,6 +278,30 @@
 	});
 }
 
+function formBattalion(entities)
+{
+	if (!entities)
+		return;
+warn("Selection panel helper form-function called!");
+	g_Battalions.formBattalion(entities);
+	g_Battalions.update();
+
+	Engine.PostNetworkCommand({
+		"type": "formation",
+		"entities": entities,
+		"name": "special/formations/battle_line"
+	});
+}
+
+function disbandBattalion(battalions)
+{
+	if (!battalions)
+		return;
+warn("Selection panel helper disband-function called!");
+	g_Battalions.disbandBattalion(battalions);
+	g_Battalions.update();
+}
+
 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
@@ -1095,6 +1095,58 @@
 		},
 	},
 
+	"formBattalion": {
+		"getInfo": function(entStates)
+		{
+			// Return false when multiple types of units are selected.
+			let refTemplateName = entStates[0].template.selectionGroupName || entStates[0].template;
+			if (entStates.length < 2 ||
+				entStates.some(entState =>
+					entState.template.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"
+			};
+		},
+		"execute": function(entStates)
+		{
+			if (entStates.length)
+				formBattalion(entStates.map(entState => entState.id));
+		},
+	},
+
+	"disbandBattalion": {
+		"getInfo": function(entStates)
+		{
+			// Only return false when no battalions are selected.
+			if (entStates.every(entState =>
+				g_Battalions.isPartOfBattalion(entState.id) === false
+			))
+				return false;
+
+			return {
+				"tooltip": colorizeHotkey("%(hotkey)s" + " ", "session.disbandbattalion") +
+				           translate("Disband all currently selected battalion(s)."),
+				"icon": "cancel.png"
+			};
+		},
+		"execute": function(entStates)
+		{
+			// Pass the battalion IDs to the function. ToDo.
+			if (entStates.length)
+				disbandBattalion(entStates.map(entState => {
+					let bat = g_Battalions.isPartOfBattalion(entState.id);
+					if (bat !== false)
+						return bat;
+				}));
+		},
+	},
+
 	"garrison": {
 		"getInfo": function(entStates)
 		{