Hearts of Iron III allows the player a fully modular division-building experience, where he or she can build a division from a wide variety of brigade components, and even mix and match them later, as needs require.
Armoured - Armoured units are comprised mainly of tanks, though they may have lighter units attached. There are different types of armoured units, from light armour to super-heavy armour.
Mountain - A mountain brigade is specially trained to fight in mountainous conditions, and they get special combat bonuses during combat in their preferred environment; rather, they don’t have a penalty, whereas everyone else does.
Infantry - The backbone of any World War II army was its ground-pounding infantry – the “boots on the ground” – who might not be very specialized, but were the core of the fighting forces. Infantry brigades were often paired with specialized brigades, such as artillery or armoured cars. Infantry stationed away from the front might be paired with military police or anti-aircraft brigades.
Cavalry - Up until just before World War II, many of the world’s armies considered horses, not tanks, to be their highly mobile forces of maneuver. World War II saw a small number of cavalry battles, some of them infamous, but in the mid-1930s, when HOI 3 starts off, there were quite a few operational cavalry brigades around the world. Cavalry was used much the same as armoured units were used throughout World War II – they had the maneuverability, but they also desperately lacked protection. Those armies who hadn’t realized by the time war started that cavalry was obsolete in the main theatre of combat found it out the hard way.
Militia - These often consisted of conscripted soldiers who were locally raised and trained, and possess infantry-like rifle training but not much else. These units were useful for maintaining order and for filling in when absolutely necessary, but normally could not be counted on to be professional military units.
Garrison - Like militia, these were soldiers who didn’t have the combat training or experience of front-line units, but whose duties mostly included keeping order, manning fortresses, and protecting important cities and other locations behind the lines.
Armoured Car - These brigades often performed a role somewhere between the cavalry and the armoured brigades. Useful for scouting and protected well enough so they wouldn’t get slaughtered by infantry, the AC brigades add a bit of heft to an infantry division, and add a little flexibility to an armoured division.
Motorised Infantry - This is basically an infantry brigade with the added benefit and liability of being mounted on trucks. These brigades could move more quickly than a standard infantry brigade while fulfilling the same role a little more rapidly, and whose speed could be a significant advantage in battle. The liability would be its reliance on fuel to move.
Mechanised Infantry - An infantry brigade that has been mounted on half-tracks and other lightly armoured vehicles, often with some heavier units to supplement their firepower. These relatively well-protected units were vulnerable to airpower, but otherwise were more durable than Infantry units, and had the firepower to punch through where run-of-the-mill infantry would have a tough time.
Anti-Tank - Brigades of infantry armed with large numbers of small, but high-velocity anti-tank guns which could be positioned in ambushes or along the front lines in order to take advantage of armoured targets of opportunity. Sometimes, the infantry would carry handheld anti-tank rifles, sneak up to where the enemy tanks were, and surprise them.
Tank Destroyer - A heavily armoured tank with a heavy-bore or high-velocity (or both) gun meant to punch through standard tank armour. Brigades supplied with packs of these were better off than regular anti-tank units because they were self-propelled and had armour to protect them from infantry and even from armoured attacks.
Anti-Air - Once the importance of airpower on the modern battlefield had been proven, especially toward the end of the war, many divisions made sure to have special anti-aircraft brigades in order to offer protection against preying aircraft.
Artillery - Howitzers and field artillery have long had a telling impact on the art of warfare. By the modern age, artillery was often a force in itself, and its careful application on the infantry or armoured battlefield proved very useful.
Engineer - A Brigade of specially trained engineers who are skilled at working out problems, building things quickly, and making the impossible happen; they can come in handy when facing off against fortresses and rivers alike. Engineers are great on defence, too, by providing bonuses to entrenchment. Engineers can add to the overall speed of a division, and they can especially help during river crossings.
Military Police (MP) - Military Police are a more inexpensive unit to produce than garrison or militia, because they aren’t trained for combat, but they are more effective than those other units at opposing and suppressing partisans and rebels.
Marine - Infantry units skilled at operation at sea and amphibious invasions. These were the mainstay of Pacific battles on both sides, but particularly on the United States side, where the marines were always the first units to go in and secure a beachhead.
Paratroop - A light-infantry brigade specially trained to parachute into a combat area, and surprise the enemy with a force behind their lines. Paratroops are generally very light in combat, and therefore can be vulnerable. It’s hoped these units would be deployed in a way so that they can be quickly supported after the initial surprise wears off.