Warcraft III: Reign of Chaos (War3 or WC3 or RoC) is a real-time strategy computer game released by Blizzard Entertainment on July 3, 2002 (US). It is the second sequel to Warcraft: Orcs & Humans, and it is the third game set in the Warcraft Universe. An expansion pack, The Frozen Throne, was released on July 1, 2003 (US).

Warcraft III contains four playable races: Humans and Orcs, which had previously appeared in Warcraft: Orcs & Humans and Warcraft II: Tides of Darkness, and the Night Elves and Undead, which are new to the Warcraft mythos. Warcraft III's single-player campaign is laid out similarly to that of StarCraft (another Blizzard game), being told through all four of the game's races in a progressive manner. Multiplayer mode allows for play against other people, via the internet, instead of playing against computer-controlled characters as is done in the single-player custom game mode. Due to the dual storylines of the previous Warcraft games, the story can only be understood if using the proper storylines of one of the campaigns in the previous games, being the Orc Campaign on Warcraft: Orcs & Humans and the Human Campaigns on both the Warcraft II: Tides of Darkness and Warcraft II: Beyond the Dark Portal.

The game proved to be one of the most anticipated and popular computer game releases ever, with 4.5 million units shipped to retail stores and over one million units sold within a month. Warcraft III won many awards including "Game of the Year" from more than six different publications.

The events played out in Warcraft III eventually set the stage for Blizzard's first MMORPG, World of Warcraft, and its subsequent expansions.

A game of Warcraft III takes place on a map of varying size, such as large plains and fields, with terrain features like rivers, mountains, seas, or cliffs. In Campaign mode, the map is initially covered with the Black Mask, a dark layer which obscures the landscape beneath until it is explored. The Black Mask, once gone, is permanently removed. Areas that have been explored but no longer are within sight range of an allied unit or building are covered with the fog of war. Though terrain remains visible, changes such as enemy troop movements and building construction are unseen. During a game, players must establish settlements to gain resources, defend against other players, and train units to explore the map and attack enemies (computer controlled foes). There are three main resources that are managed in Warcraft III: gold, lumber, and food. The first two are required to construct units and buildings, while food restricts the maximum number of units the player may control at one time.

The game also introduces creeps, computer controlled units that are hostile to all players. Creeps guard key areas such as gold mines or neutral buildings and, when killed, provide experience points, gold, and special items to a player's hero. Warcraft III also introduced a day/night cycle to the series. Besides having advantages or disadvantages for certain races, at night most creeps fall asleep, making nighttime scouting safer; however, the line of sight for most units is also reduced. Other minor changes to the gameplay were due to the 3D terrain. For instance, units on a cliff have an attack bonus when attacking units at lower elevations.

In previous Warcraft games, there were only two playable races, Orcs and Humans, which had more similarities than differences. Barring cosmetic changes, most Orc units were identical to their Human counterparts. In Warcraft III, the Night Elves and the Undead are added as playable races. Additionally, as in StarCraft, each race has a unique set of units, structures, technologies, and base-building methodology.

In addition, Warcraft III adds powerful new units called heroes. For each enemy unit killed, a hero will gain experience points, which allow the hero to level-up to a maximum level of 10. Progressing up a level increases the heroes attributes and also allows the hero to gain new spell options (bringing RPG elements to the series). Certain hero abilities can apply beneficial auras to allied units. All heroes can equip items to increase skills, defense, and other abilities. At level six, the hero can obtain an "ultimate" skill that is more powerful than the three other spells that the hero possesses. Heroes can also utilize the various natural resources found throughout the map, such as controllable non-player characters, and markets in which the hero can purchase usable items. Often, the playing style of ones hero units (it is bound up to three at one time) decides who wins or loses the match/battle.

Warcraft III's campaign mode is broken up into four campaigns, each featuring a different race which the player controls. Each campaign is itself divided into chapters, which are like missions. Unlike previous Blizzard titles, such as Warcraft II or StarCraft, players are not directed to mission briefings in which plot exposition occurs and objectives are announced; rather, Warcraft III uses a system of "seamless quests." Some plot development happens in an occasional cinematic, but most occurs in-game with cutscenes. Objectives, known as quests, are revealed to the player during the progress of the map. Main quests are those that the player must complete to proceed to the next chapter, but there are also optional quests which are not initially revealed, but can be discovered and completed alongside the main objectives.

Through each race's campaign, the player retains control of one or more heroes, which slowly grow in experience as the levels progress. This experience is carried over to subsequent missions, allowing the hero to grow throughout the course of the campaign.