The player usually starts with a single HQ building and a few basic units. Gameplay is initially focused on base and unit construction, then shifting to capturing and holding strategic locations on the battlefield. These control points provide resources to construct additional units and buildings or unlock new technology. Single player campaign battles are mostly won by destroying the enemy base(s).
There are two primary resources: Requisition and Power. These resources are not harvested or otherwise gathered by the player's units.
Some Requisition is generated the army HQ, and the player can increase his Requisition income by capturing and controlling Strategic Points on the battlefield. These locations can be reinforced with defensive structures that further increase the rate of Requisition income. Resources are unlimited, but sources of Requisition will eventually decay, their income rate decreasing. Power is gathered by building Plasma Generators, each HQ supporting up to six. Some maps have Slag Deposit sites where more powerful generators can be constructed. Basic units usually require little Power and advanced units lots of it.
In addition, the Orks have an Ork Resource. Ork Resource is generated by Waaagh! Banners and is used up when creating Ork troops and vehicles. The number of banners and the size of the Ork population determines the Ork's Waaagh! level which in turn determines the technology level the player has access to.
Infantry units are controlled as squads rather than individuals. Most fully reinforced squads consist of about 10 units. The squad moves and attacks as a single entity. Squads can produce and replace their own units anywhere in the field. They can be equipped with special weapons and have heroes or special units attached to them. For example, a Space Marine Squad starts with 4 Marines, but can be reinforced to 8 Marines and a Sergeant. Individual marines can be equipped with heavy weapons. The player is free to choose particular upgrades and can specialize each squad for a specific purpose.
Vehicles are highly resistant to anti-infantry weaponry, so they must be targeted with specific anti-vehicle weapons (e.g., missile launchers) to be destroyed. Most vehicles can also be upgraded with multiple weapon systems, usually differentiating between anti-infantry and anti-vehicle armaments.
In addition to a typical health mechanic, infantry squads also have morale points. When morale drops to zero, the squad "breaks" and their fighting ability is significantly reduced. The squad's morale will regenerate while the squad is not in combat. Some weapon types, such as flamethrowers, are made specifically to demoralize the enemy. Favorable terrain gives units a defensive cover bonus, while water and swamps slow units down and decrease their defensive ability.
The game is set in the Warhammer 40,000 universe, a dystopian vision of the future with gothic undertones. Humanity has forged a galaxy-spanning empire, The Imperium, with the Immortal Emperor as humanity's leader and God. The Imperium is in a state of constant war with the Orks, Eldar, and the human servants of Chaos. The single player campaign is set on the planet Tartarus.
Warhammer 40,000: Dawn of War features four playable armies.
The Space Marines are the elite genetically modified superhuman soldiers of the Imperium. Space Marines have the highest morale in the game. Their troops and vehicles are more expensive, however. The Space Marines are the only playable race in the single player section of the game, and the 11-mission campaign features the Blood Ravens chapter as the protagonists, led by Captain Gabriel Angelos and Librarian Isador Akios.
The Chaos Marines are traitors that have chosen to abandon The Imperium and their duties to humanity, and instead worship the Gods of Chaos. In Dawn of War, they use troops that are similar to their Imperium counterparts. In addition, they employ morale-draining daemons and Psykers. The campaign features the Alpha Legion, with their leaders, Sindri Myr and Lord Bale.
The Eldar are an ancient, technologically advanced race. Eldar in Dawn of War are fast and fragile. They are able to move quickly across the map via webway gates. The campaign features the craftworld of Biel-Tan, and their leader, Farseer Macha.
The Orks are a savage, warlike race. In large enough numbers, they gain morale immunity. The tech tree for Orks differs from the other races in that it depends on the amount of orks currently in your army and the number of erected WAAAGH!! banners. Several clans are featured in the campaign, as is the Ork Warboss, Orkamungus.
In addition to the four fully playable races, the Imperial Guard also make appearances in the single player campaign as allies of the Space Marines, led by Colonel Brom. They would later be made into a featured race for the first expansion pack, Warhammer 40,000: Dawn of War: Winter Assault.