Warhammer 40,000: Dawn of War/Space Marines

Space Marines are the elite warriors of the Imperium of Man. They are huge super-soldiers enhanced by numerous implants (such as a secondary heart and other neo-organs), extensive medication and hypnotherapy. These enhancements and inhuman amounts of training grant them superhuman physical and mental abilities. Cybernetic implants allow the use of their trademark power armor, a heavily armored suit that augments their strength and provides basic survival/medical functions.

Space Marines are organized in around 1,000 Chapters consisting of 10 companies of 100 men each. Each Chapter is a completely self-sufficient force with its own space fleet, support personnel and so on. They are fiercely loyal to the Emperor, but also autonomous and tend to serve as they see fit. The Imperium they serve is a massive (about 1,000,000 worlds) but thinly spread dominion locked in endless war with Chaos and numerous alien species. The whole society is a dystopian war machine where entire planets are dedicated to weapon factories and the average citizen's life is barely better than being killed by Orks.

The Space Marines are a small force of extremely skilled and tough individuals. In addition to elite infantry they have good vehicles and orbital support. If you don't mind a little fanaticism, there's nothing not to like.

Force Commander (Gabriel Angelos)
The Force Commander is your main commander unit, available through every mission. He is an uber-veteran with superior combat skills. He has good ranged damage and great melee damage, and can be upgraded with better weapons and armour. The Force Commander can also order Orbital Bombardment strikes (requires the Orbital Relay structure) which do high damage and knock infantry to the ground. Attached to a squad, he provides a big morale bonus.
 * Class: Commander
 * Role: Melee specialist, air strikes

In low-level fights, you can send the Force Commander in solo; his melee attack will disrupt infantry squads while your ranged units shoot them up. He will even do decent damage against vehicles and structures. If the enemy has many advanced units, you should be more careful and attach him to a squad. If you lose him, he will respawn around your Stronghold for free after a while, so just use and abuse him.

Librarian (Isador Akios)

 * Class: Commander
 * Role: Melee specialist, psyker

Inquisitor Toth

 * Class: Commander
 * Role: Melee specialist, air strikes

Servitor
The Servitor builds your structures and repairs your vehicles, and that's about it. You can set two on the same project to make it happen faster. Servitors take up Squad Cap, so don't keep unnecessary units around. Note that you can Deep Strike them through the Orbital Relay (can be useful if setting up a secondary base).
 * Class: Infantry
 * Role: Builder unit

Scout Marine Squad
Scout Marines are light infantry that can infiltrate, making them invisible to most enemy units as long as they do not attack. They are lightly armored and have a maximum squad size of 4, so they should generally avoid fights. In single-player they are mainly used to scout enemy positions and spot targets for long-range firepower. When upgraded with sniper rifles they can also take out leader units and destroy enemy morale from behind your lines in bigger scraps. You should get Scout Infiltration Research from the Stronghold right away to make them less vulnerable (besides, it's pretty cheap).
 * Class: Infantry
 * Role: Scouting, sniping

Space Marine Squad
The Space Marine Squad - Tactical Squad in the tabletop game - is your basic infantry unit. They hold their own in both ranged and melee combat; mostly you want them shooting but sometimes charging at ranged specialists. They can be upgraded with a variety of heavy weapons, making them useful in all situations. Upgrading one squad with only one type is best for managing your Marines (i.e., correct range and correct targets).
 * Class: Heavy infantry
 * Role: Versatile


 * Flamers have short range and moderate damage, but will quickly wreck enemy morale, making infantry very ineffective.
 * Heavy bolters have long range and great damage against infantry, but cannot be fired while moving and require a short setup time to fire.
 * Plasma guns have shorter range but can be fired on the move and are more effective against heavy infantry and commander units.
 * Missile launchers have very long range and good damage against vehicles and buildings. They do little damage against infantry, however they will disrupt and demoralize.

There are several global upgrades that apply to Space Marine Squads. You want all, probably starting with the Bionics and Heavy Weapon Increase upgrades. Each squad can be upgraded with a Sergeant who increases squad morale and has the Rally special ability (instant full morale) which should be used a lot. Also get the Frag Grenades upgrade and frag ranged squads to disrupt them.

Assault Marine Squad
Assault Marines are jump infantry, meaning they can jetpack over impassable terrain. They are melee units, most effective when jumping on enemy ranged specialists. Because of their mobility they can quickly reinforce a weak spot in a fight or prevent an enemy squad from escaping. Assault Marines can be equipped with short-range melta bombs so they can jump in, bomb a vehicle or building, and jump out. You can also jump them into an enemy base to spot good targets for artillery or a place for some Deep Striking.
 * Class: Heavy infantry
 * Role: Melee anti-infantry

Terminator Squad
These are the most senior veterans of an already elite army, equipped with a heavier version of power armor. The squad can carry 2 vehicle-class heavy weapons; they are slow but fire on the move and also teleport (with a delay). They are excellent - the only drawback is that they take 4 Squad Cap, meaning you could get 2 Tactical Squads (8 heavy weapons) instead, so you must weigh whether they are worth it. In cramped surroundings where it's difficult to fit many squads, definitely - in many other cases, probably not.
 * Class: Heavy infantry
 * Role: Ranged firepower

Note: When you buy Terminator Squads, they come teleport-ready but without reinforcement or upgrades. It's usually best to Deep Strike them inside your own base, get the upgrades, then put them back in the Chapel-Barracks for full-strength insertion.

Assault Terminator Squad
These are a close-combat version of regular Terminators, effective against everything. They take 4 Squad Cap as well, but replacing them with 2 Assault Marine Squads isn't the same - these guys have crazy health and beat even vehicles to scrap. Throw in the teleport with decent range (and Deep Strikes from your base) and they can really stir up trouble. Attach a commander unit to them for best results.
 * Class: Heavy infantry
 * Role: Melee specialist

Apothecary
The Apothecary can be attached to a squad like a commander unit; he doesn't improve the squad's morale or fighting capacity, but boosts the squad's health regeneration (and that of other nearby units). He has a chainsword and can hold his own in melee, but he's not really a combat bonus like a commander. There's no reason not to get and attach the maximum number (4) of Apothecaries.
 * Class: Heavy infantry
 * Role: Healer unit

Rhino
It's an unarmed transport - you put guys inside and drive around. In theory it's very smart as it can nullify lots of anti-infantry fire and you can move your infantry around faster. In practice it seems pretty useless (in campaign play). Build armed vehicles instead and tell your guys to walk.
 * Class: Vehicle
 * Role: Transport

Land Speeder
The Land Speeder is a light vehicle that can jump (like Assault Marines) and deal out a good amount of anti-infantry fire. If the enemy has no anti-vehicle capacity, even one of these could be a pain. For example, take one to scout around and kill any lone infantry squads. Mass several and jump them to an enemy base through the back for some sabotage. They are fried pretty quickly by anti-vehicle weaponry, however, so heavier vehicles are preferable most of the time.
 * Class: Vehicle
 * Role: Anti-infantry

Dreadnought
The Dreadnought is your walker vehicle, a war machine operated by a horribly wounded Space Marine veteran permanently implanted within. It is heavily armored and comes with two melee weapons. Don't get the ranged weapons, the Dread does huge melee damage and you have tons of other ranged units already. Put it in front to draw useless anti-infantry fire or (the best part) drop it right on the enemy from the Orbital Relay. Always have some.
 * Class: Vehicle
 * Role: Close combat

Hellfire Dreadnought
This is a ranged Dreadnought variant with no melee capacity. It's less durable and a bit cheaper. You probably won't want this; you already have ranged infantry, and if you want a vehicle with ranged weapons, get a Predator tank instead.
 * Class: Vehicle
 * Role: Ranged combat

Whirlwind

 * Class: Vehicle
 * Role: Missile artillery

Predator

 * Class: Vehicle
 * Role: Main battle tank

Land Raider
Yes, it's as cool as it sounds. It's a behemoth tank effective against everything and it can also transport one squad, including Terminators, who can't fit in normal transports. The only problem is the Vehicle Cap cost of 5, which means you can almost get 2 Predators instead. Still, you probably want one of these anyway... The Land Raider is your Relic unit, meaning you have to control a Relic on the map in order to build them.
 * Class: Vehicle
 * Role: Super-heavy tank/transport

The Land Raider has two twin-linked lascannons, so it's most efficient to use it as a tank killer. Make use of the transport capability; load an infantry squad (remember it doesn't have to be Terminators, a Tactical Squad can be powerful too) or, alternatively, bring along a Servitor to repair your tanks. Also remember to activate the Machine Spirit ability (-40% damage) when drawing fire.

Heavy Bolter Turret
Well, it's a box with heavy bolters. Decent against infantry, can be upgraded to missile launchers so it's decent against vehicles. If you really need them to defend something, build several and mix both types (missile launchers in the back row as they have longer distance). Even if the enemy has no vehicles, have some missile turrets - they will knock infantry around and your bolter turrets will pick them off.
 * Class: Building
 * Role: Stationary defense

Orbital Relay
This structure allows you to Deep Strike most infantry units and Dreadnoughts and enables the Orbital Bombardment special ability. You walk your units inside and after a delay you can drop them anywhere on the map (that's not covered by fog of war). The obvious tactic is using it to drop melee units on/behind enemies but of course you can put a ranged squad in a good position or use it for transportation on large maps.
 * Class: Building
 * Role: Deep Striking