Hearts of Iron IV is a
grand strategy wargame that revolves around
World War II. The player may play as any nation in the world, starting on January 1st, 1936, or August 14th, 1939 start dates in
single-player or
multiplayer.
Economy and "political power" Equipment is produced by military factories, while ships are built by
dockyards. These military factories and dockyards are constructed using civilian factories, which also can construct a variety of other buildings, produce
consumer goods for the civilian population, and oversee
trade with other nations. Most nations are initially forced to devote a significant number of their civilian factories to producing consumer goods, but as the nation becomes increasingly
mobilized, more factories will be freed up for other purposes. Since the update coinciding with the expansion
No Compromise, No Surrender, all factories need to be powered by
coal, and power gain from coal can be influenced by technologies and other buildings in states such as
dams or
nuclear power plants. With the expansion
Arms Against Tyranny, the player can interact with the global
arms market, as well as gain access to
military industrial organizations, which improve military equipment over time. Mobilization,
conscription, and
trade laws are represented as policies that the player may adjust with the proper amount of
political power, an abstract resource that is also used to appoint new
ministers and change other parts of the nation's government.
Military and combat A nation's
military is divided between
ground forces,
naval forces, and
aerial forces. For the ground forces, the player may train, customize, and command
divisions consisting of various types of
infantry,
tanks,
cavalry,
motorized, and
mechanized battalions. These divisions require equipment and
manpower to fight properly. The navy and air force also require men and equipment, including the actual
warships and
warplanes that are used in combat. The tanks, airplanes, and boats could also be manually customized by the player. Land in the game is divided into tiny regions known as provinces (also called tiles), which are grouped to form
states. Each state has a certain amount of shared and state building slots, both of which affect the whole state, while provinces have province building slots that only impact the individual province. Similarly, major seas and oceans (for warships) and the sky (for warplanes) are divided into strategic regions. Sea regions and provinces each have a type of
terrain and
weather assigned to them that determines how well different types of units will perform in combat there. Air regions do not have terrain types but still have weather. Divisions are deployed in provinces and can capture enemy provinces and engage in combat. How well divisions perform in combat depends on various factors, such as the quality of their equipment, the weather, the type of terrain, the skill and traits of the
general commanding the divisions, aerial combat in the region, supply lines, their organisation, how high their defense or attack—'soft' or 'hard'—is, and supporting units. Technologies can be researched to improve equipment. Military doctrines can be upgraded by using army XP, among other things, which often means that a more technologically advanced nation will have an edge in combat. If a division (or a group of divisions) successfully overwhelms an enemy province, they will
occupy it. Some provinces may have victory points, which can push a nation closer towards
capitulation if occupied. Occupying key provinces within a state allows the occupying power to access the enemy's factories and
natural resources in that state.
Resistance to occupation within a state, which occurs when a nation does not have a 'core' on a controlled territory, can hamper the occupying power's control over it. Nations can develop
nuclear bombs if they have the proper technology, which can be used to devastate enemy provinces and states. Since the release of the update coinciding with the expansion
Götterdämmerung, special projects have been added which require facilities to be constructed to allow for the pursuit of nuclear energy and weapons, as well as other technologies such as
super heavy tanks, intercontinental
bombers, or
rockets.
Politics, diplomacy, and autonomy Hearts of Iron IV also attempts to recreate the complex
diplomatic relationships of the late 1930s. Nations may undertake a variety of diplomatic actions; they may sign
non-aggression pacts,
guarantee the independence of other nations, and grant or request
military access, amongst other things. Another key feature of diplomacy is the ability to create a
faction or invite other nations to an existing one. Factions represent the main alliances of the era, like the
Axis and
Allies. For gameplay purposes, real-world factions like the Axis and Allies are split into numerous smaller factions, like the
Comintern, the
Chinese United Front, the
American Commonwealth (which later joins the Allies), and the
Greater East Asia Co-Prosperity Sphere. Faction members may assist each other in wars, making faction members precious assets. The expansion
No Compromise, No Surrender introduced new faction mechanics in addition to the previous listed, such as faction goals, rules, research, military operations, intelligence, initiative, and influence. Other diplomatic actions are also available, such as
coups,
war justification, and influencing other nations with their
ideology. Countries in the game may be
democratic,
communist,
fascist, or
non-aligned. Each of the four ideologies has advantages and disadvantages; for example, fascist nations can go to war with other countries easily, but other nations are not as willing to trade with them. On the opposite end, democratic nations have difficulties in invading other countries, but they can guarantee the independence of any nation with greater ease. If a nation's ideology is democratic, it may hold
elections which can allow for party popularity to shift without the use of political power. If a nation's ideology is non-aligned, then it can be several things, such as a
monarchy,
dictatorship, or an
anarchy. For example, if the German Reich opposes Hitler, it is possible to bring back the
German Empire (
monarchy), which is non-aligned. Accordingly, if the
anarchists win the
Spanish Civil War, they are non-aligned as well. If a different ideology becomes too popular in a country, a
referendum may be held that will peacefully convert the nation to the most popular ideology. Otherwise, ideologies may come to power violently through coups, civil wars, or
forced subjugation by a foreign power. The expansion
Together for Victory introduced a new autonomy system for subjects and their overlords which allow for overlords to decrease the autonomy of and annex their subjects, while subjects can increase their autonomy and become independent. This is influenced through a points system, which can be affected by
Lend-Lease, building in subject by overlord, national spirits, and more. Different autonomy types influence what bonuses the overlord gets from their subjects, such as factory percentage or extra trade. The autonomy system for most countries includes the following autonomy types:
Free;
Dominion;
Colony;
Puppet;
Integrated Puppet; and
Annexed. Subsequent DLCs have added more autonomy types, such as
Reichskommissariats,
personal unions,
EU member states, and
Chinese warlord subjects. This diplomacy is further expanded through the addition of
espionage in the expansion
La Résistance, which extends gameplay by adding the ability to do espionage in other nations and counterintelligence at home.
National focuses and spirits While
Hearts of Iron IV does feature some
scripted events, the game features a "national focus" system that makes fixed events less necessary than in previous installments in the series. Each country in the game has a "focus tree" with various "national focuses" that can achieve a variety of in-game effects, such as granting certain effects or triggering certain events. For example, for Germany to complete the national focus "
Anschluss",
Germany must first complete the previous focuses that branch down to the Anschluss focus. Meanwhile, other focuses can grant special bonuses, like faster research times for certain technologies or extra factories. While some bonuses (like extra factories) are very tangible, others (like improvements to morale) are more abstract. These abstract bonuses are represented by "national spirits" that can be temporary or permanent. Not all national spirits are granted by focuses, and not all spirits are entirely beneficial. Focuses are completed over time; only one focus may be worked on at once, and working on one consumes some political power. Initially, only a handful of key nations, like Nazi Germany, the United Kingdom, the Soviet Union, and the United States, had unique focus trees; all other nations shared a generic one. Subsequent updates and
DLCs have added focus trees to other nations as well.
Events and decisions Country events are pop-ups that can be triggered by national focuses, decisions, and other events (visible or hidden). Each event gives the player several options to pick from, often granting either bonuses or negatives (such as selecting leaders in an
election), while also sometimes being used as a
notification (informing players that a
civil war is about to begin, for example). News events are another feature that allow players to be notified when events of international significance occur. Decisions were a feature added in the update coinciding with the expansion
Waking the Tiger that allow players to select policies without using their national focus slot. They often cost political power, and can be used for actions such as
propaganda,
resource development,
state integrations, country formations, and other policies. ==Development==