To understand the game mechanics of land combat, visit the land combat guide.
First and foremost, it is important to understand that the principles of war in Vicky2 are based on attrition warfare: there is relatively little room for strategic maneuver and tactics, thus the primary game mechanism is to have superior resources than the opponent. Defensive war is generally the most effective type of war to wage in the game. An effective way to manage a war is to play defensively, fighting initial battles in home territory or in an enemy border province. This will help reduce the military force of the enemy, while creating a good starting war score. Defensive war can provide terrain bonuses, fort bonuses, and units suffer far less attrition. When enemy forces are effectively reduced to a manageable size, invasion of the enemy's home territories becomes possible.
Force Composition
Unit Composition: As a general rule, a mixed army will produce the most successful results. Specifically, armies should have an even mix of infantry and artillery. This is necessary because artillery (and later airplanes) are the only units that can occupy the second line of frontage and still engage in combat. Thus, infantry in the second line serve no purpose in combat until they get into the first line. Some players suggest adding slightly more infantry than artillery with the idea that infantry will die off and artillery in the front lines will be decimated. Since calvary can attack diagonally, they are most useful in pairs for any army: one regiment for each flank.
Size: The size of an army obviously depends on soldiers POPs, but a wise player will build armies that consider three other key factors:
- Supply Limit: An army that exceeds the supply limit of terrain it occupies can suffer significant losses due to attrition. Therefore, armies should be built to a size using the Supply map mode to ensure any region they occupy will not be below their supply limit. Note, however, that armies do not suffer attrition during combat. It can therefore be advantageous to send an enormous force into battle, so long as they quickly move into a safe supply limit territory when the battle is over.
- Frontage: The number of units that can engage in combat are limited, which varies over time as technology is researched. In the end game, in Mountains for example, only 3 regiments (e.g. a maximum of 9,000 soldiers), can fight on the front line of the battle with 3 units (preferably artillery) in the rear. In this sense, a strong unit of 3 regiments of infantry and 3 regiments of artillery (18,000 total soldiers or an "18" stack size) provides for excellent mountain defensive units. For plains, the end-game combat width is 10, allowing a total of 20 regiments to engage in combat. Thus, the maximum effective size for combat units in plains is a stack size of 60 (60,000 soldiers).
- Management: For ease of management, some players prefer to build small, modular stacks of armies (for example: 3 infantry and 2 artillery) that can then be deployed en masse for particular battles. This helps manage the effects of attrition. As supply limits increase through technologies, these sizes can increase, but the modular approach remains.
Attrition
Attrition warfare is an important component of strategic game play on a number of levels. For reinforcements, it is better to fill the combat width and have all reserve troops in a safe territory outside of combat. This allows reserve units to reinforce and gain strength, which is ideal since they are not able to help combat anyway. When the main army begins to loose org and frontage spaces open, the reserves can be sent in.
Faction-level organization and mobilization strategy
The size of various armies, and their unit composition, cannot be considered in a wholly hermetic and isolated context. The player must form their armies, mobilize them, and deploy them in response to highly faction-specific demands and pressures. For these purposes, there are three possible ways to integrate mobilization into a player's overall land combat strategy, namely: the formation of a wholly professional army with no dependence on mobilized pops, the formation of a wholly mobilized army with total dependence on mobilized pops, and the formation of armies with professional and mobilized elements.
Wholly professional armies do not depend on mobilized pops for their force composition. In this model, all three parts of an army - the infantry, the cavalry, and the artillery - are recruited exclusively from 
Soldier pops. Because this model presupposes the existence of completed armies before a conflict breaks out, this model guarantees a high level of force readiness. A player can swiftly respond to emergencies, and doesn't have to wait months for pops to mobilize.
On the other hand, this means that the player is constrained in their force numbers and force projection, since they can't allow their Soldier pops to specialize in any one particular unit type. This is especially true if a player has a low number of Soldier pops to recruit from. Similarly, the low number of soldiers means that even with perfect force optimization, against more numerous opponents, the player is limited to short and fast campaigns. A more numerous opponent can pay the price of attrition, but the player can't. They must swiftly achieve their war goals before an opponent is able to muster more numerous forces, and get good at fighting at a numerical disadvantage.
This model of organization is useful for smaller countries that are surrounded by much more powerful neighbors, like 
Belgium, 
Netherlands, 
Portugal, 
Serbia, 
Greece, and 
Switzerland. In each of these cases, the metropolitan territories of these countries are rather small, and none of these countries can afford to wait for Farmer, Labourer and Craftsman pops to mobilize in response to an enemy invasion. For those players, it's better to have an army of 30,000 immediately, rather than an army of 60,000 that takes forever to mobilize.
Wholly mobilized armies are highly dependent on mobilized pops for their force composition. In this model, the cavalry and the artillery are recruited from Soldier pops, while the infantry are recruited from mobilized pops. Because this model allows for the Soldier pops to specialize in particular unit types, and uses mobilized pops for infantry, it allows the player to recruit and field much larger armies than their soldier pops would suggest. Therefore, a player who uses a wholly mobilized army will be able to overwhelm an opponent using a wholly professional army, all other things being equal.
On the other hand, this means that the player is highly dependent on mobilization speed, and discovering war justification attempts early. If they fail to discover war justification attempts early, it means that some time after a conflict begins, the player will be totally unable to respond to enemy invasions - until they are able to mobilize and organize enough pops to send their now-completed armies into battle. Furthermore, they will also pay a price in reinforcement speed, because mobilized units reinforce only half as fast as those from Soldier pops.
Therefore, a player seeking to implement this strategy must be a) willing to lose some provinces to enemy occupation, b) have a layer of buffer states between themselves and the enemy, or c) have few external threats from other factions. This means that it's useful for factions like the 
USA, the 
United Kingdom, 
Russia, 
China, 
Japan, and 
Brazil. These countries control a lot of territory, can afford to lose a few provinces to enemy occupation, are generally isolated from foreign threats, and can use their immense manpower to eventually crush whatever army invaded them.
A compromise can be reached with the creation of armies that feature both professional and mobilized elements. In this model, some of the player's armies rely on Soldier POPs, while some rely on mobilized POPs. Because this model allows for professional and mobilized armies to exist within the same faction, it means that the player retains some ability to immediately respond to petty wars, invasions, and emergencies, while also allowing them to cash in on the numerical advantages that mobilization gives them should they need to fight against a much more numerous opponent.
Unit selection Before a Heart of Darkness
Infantry: This will make up the backbone of any army throughout most of the game. As a general rule, any army should be approximately 50% infantry.
Guards: These can be used in place of infantry, but they are very expensive to supply. Thus, while slightly better than infantry, some consider them not worth the expense.
Artillery: A very powerful unit throughout the game, particularly in later stages. Artillery should be approximately 50% of any given army. In the late game, some players suggest 100% pure artillery armies, though some consider this gamey.
Cavalry: Best in the early game against uncivilized nations. In mid-game, they are only useful in limited numbers in an army (possibly up to 4 regiments max), or as stand alone units set to hunt rebels. In the late game, when frontages are filled, they serve no special purpose.
Engineers: Only useful as a single unit in an army to speed up occupation.
Tanks: An expensive unit that has a lot power and good sieging ability(Basically a powered up Artillery).
Airplanes: Very expensive units for maintenance, but have excellent reconnaissance values and are thus useful as single units in occupying armies.
Unit selection in Heart of Darkness
In Heart of Darkness there were some major rebalancing of the units.
Irregulars and Cavalry: Only uncivilized nations should bother with these, and even then only because they don't have the tech for Infantry
Dragoons / 
Hussars, since Irregulars have only about half the stats of Infantry and Cavalry is also flatly inferior to Dragoons and Hussars. Cavalry is tolerable until westernization (they're essential for their reconnaissance), but Foreign Weapons is an important reform that will give a substantial military advantage over other uncivs.- Infantry: Infantry's defense has been increased, making this unit well-suited for defense or offense. This will make up the backbone of any army throughout most of the game. As a general rule, any army should be approximately 50% infantry.
- Guards: Guards are rebalanced so they are no longer considered an upgrade to Infantry. They are stronger than infantry on the attack, but will not fight as well if their army is attacked and they must defend. They are considered the early game main breakthrough unit so some players replace one or two infantry units with guards. They can only be recruited from accepted culture pops. Their effectiveness lessens as Tanks are unlocked.
- Artillery: A very powerful unit throughout the game, particularly in later stages. Unlike previous versions of the game, Artillery in HoD has extremely low combat stats, but very high support, meaning it can only be effective in the back rank. The days of "All Artillery Armies" are gone.
- Dragoons: These are the Cavalry counterpart for an army of civilized countries. You can consider them mounted Infantry for all needs and purposes. They also have a reconnaissance stat that will help during occupations in the early game, and will fill the role of a combat recon unit until you unlock Airplanes. For the pure reconnaissance role Hussars are strictly better, but much more expensive (Luxury Clothes is a scarce resource early on).
Cuirassiers: They have their own niche and use as well. Their field of specialty is glorious charge. They were rebalanced into an early tank-like unit. They are perfectly capable of riding down early game infantries, but their superior power has an expiration date as Dragoons get better combat stats from inventions.- Hussar: This is the advanced and specialized type of elite light cavalry. They are pretty expensive. You can find Hussars on the flanks of armies of richer and more powerful countries, where they replace Dragoons. They also have a high Reconnaissance, making them great during occupations and against dug-in enemies, but their combat stats will suffer as Dragoons get better inventions.
- Engineers: Unlike earlier versions of the game they are now not only great during sieges but they also fight from the back rank (at half strength, increased up to full strength with Army Doctrine tech) thanks to their new support stat. Keep at least one or two of these in your armies if you plan on occupying provinces with forts. Optimal occupation efficiency comes with a ratio of one Engineer brigade to nine other brigades. They are, however, still not as powerful in rear-rank combat as artillery, so many players just use a few engineer divisions to speed up occupations as and when needed.
- Tanks: In the late game, Tanks become your main breakthrough and siege units. Tanks are multi-purpose, taking on the dual role of Guard and Engineer, though also putting a huge strain on your budget when overused. They no longer have the discipline bonus of previous versions, but their ability to fight in the front rank has been upgraded with better attack stats along with the same siege stat as Engineers.
- Airplanes: These are now a support unit, fighting from the second line in battle. If you can afford them, they will greatly add to your army's reconnaissance. They will allow you to faster conquer provinces and are good in combat as a support unit. Airplanes have been upgraded with superior defense over their previous versions, but still can't hold up a sustained assault if used in the front line. As they are primarily used for capturing land, avoid replacing all your superior Artillery with Airplanes until you unlock all aviation inventions; a handful will do just fine. Once you unlock enough inventions for Airplanes, their defensive stat jumps to 30 with a support stat of 600%, allowing them to be the ultimate defensive unit in both the front rank and the back rank. In Patch 3.04 they are rebalanced to reduce their discipline, making them less effective defending with no front line support.
Leaders
The best combat leaders have good quality Leader traits. Among the best leader traits for combat effectiveness are:
- Defensive bonus: Arguably the most valuable bonus in a leader, these traits are relatively rare. Maximum possible value is 6.
- Organization bonus: A relatively rare but powerful trait that is never available for defensive leaders.
- Attack bonus: Attack bonuses are somewhat common. Good for dedicated offensive units, maximum possible value is 5.
- Experience: Experience improves dice rolls as well as attack and defense scores. Experience allows small armies to trump larger ones.
Even if the leaders have negative traits, it is still better than having a leaderless stack.
Research
主条目：Research & technology
In order of priority:
- Army Leadership: Tactics are the most powerful military concept in the game, as they reduce casualties taken in combat.
- Chemistry (under Industry): Increases supply limits, allowing larger armies to be fielded without suffering attrition. (But bear in mind Light Armament tech also decreases combat width, limiting the maximum effective size of armies.) Also increases Military Hospitals, reducing pop losses from casualties.
- Heavy Armament: Artillery is generally the most powerful unit, and research here is important.
The remaining categories are more debatable and depend on playing style to some degree:
- Army Doctrine: For players who pursue defensive warfare. Perhaps ironically, this tech also speeds reconnaissance.
- Military Science: Org bonuses keep units fighting longer
- Light Armament: Attack increases are useful, also required (up to breech-loaded) for colonization as it reduces life rating. The second and third techs are needed for Engineers and Guards respectively.