Hearts of Iron IV: The Wargame That Models Everything

Most strategy games about the Second World War pick a scale and stay there: a theatre, a campaign, a single beach. Hearts of Iron IV, which Paradox Development Studio released in June 2016, picks every scale at once. A single campaign asks the player to design infantry division templates battalion by battalion, manage a research queue balancing tank armour against radar, decide how much political capital to spend nudging a nation from democracy toward authoritarianism, and then fight the resulting war across a real-time map where supply lines, terrain and weather all matter independently. It is the most systems-dense strategy game Paradox has shipped, and the ambition is both the appeal and the risk, because a model with this many interlocking parts is also a model with this many places for a player to quietly break it.

The division designer as the actual game

The headline system, and the one that separates HOI4 from every other WWII strategy title, is the division designer: rather than picking pre-built unit types, the player assembles a division from battalions and support companies — infantry, artillery, tanks, engineers, reconnaissance — subject to a combat width limit that punishes both over-stuffed divisions and undersized ones. A narrow, deep division holds a line well but attacks poorly; a wide, shallow one does the opposite. Getting this right, and tailoring templates to terrain and doctrine, is where the actual strategic skill in HOI4 lives, far more than in the direct unit-pushing that resembles a traditional RTS. A player who never touches the designer and just uses the default templates will lose to one who does, reliably, regardless of raw industrial output, because the war is won or lost in the spreadsheet before a single division crosses a border.

Political power and the slow drift toward war

Before any war starts, HOI4 asks the player to manage a nation’s internal drift: political power, spent on national focuses, advisors and ideology shifts, is the currency that decides whether a historically democratic nation stays that way or slides toward fascism or communism over the course of the 1936 to 1948 campaign window. The national focus tree — unique to each major nation and substantially expanded through later expansions — gives every country its own branching path of historical or alternate-history choices, and the “world tension” mechanic gates how aggressively a nation can act without triggering a global response, which means the build-up to war is itself a managed resource rather than a fixed countdown. A player rushing towards conflict too early finds the world unprepared to accept it; one who waits too long finds the historical window for easy territorial gains has closed. Later expansions added advisors and party popularity mechanics that make the drift itself a game within the game: a fascist Britain or a communist United States are both achievable, given enough patient focus-spending, and the game’s own achievement list openly encourages players to chase these outcomes rather than treating the historical starting alignment as fixed scenery. That openness is unusual for a game set inside a real, well-documented war, and it is the clearest sign that HOI4 treats history as a starting position to be argued with rather than a script to be re-enacted.

Supply, weather and the war that grinds

The systems that get the least attention in most discussions of HOI4 are the ones that decide most real campaigns: supply. Every division needs fuel and matériel delivered along rail and truck lines from the factories producing it, and an overextended offensive into Russia in winter, with supply lines stretched past their effective range and temperatures collapsing organisation, recreates the actual operational failure of the historical invasion without the game needing to script it as an event. Terrain multiplies the effect — mountains and rivers slow logistics dramatically, and a badly planned offensive through the wrong province can stall an otherwise superior army for months of game time. It’s unglamorous modelling, entirely invisible until it isn’t, and then it’s the reason an offensive that looked unstoppable on the map screen quietly ran out of fuel two hundred kilometres short of its goal. Rail capacity into a newly conquered region takes real in-game time to build up, which means even a swift, well-executed encirclement can stall afterwards simply because there isn’t yet enough infrastructure to feed the divisions holding the new line, a detail that rewards players who plan their logistics network a full campaign phase ahead of their tanks.

Naval and air, the neglected fronts

Land combat gets the design attention in most discussions of HOI4, but the naval and air layers run on their own logic and punish neglect just as harshly. Naval combat plays out as scripted engagements between fleets built from custom ship designs — hull, armour, guns, and later aircraft carriers assembled the same modular way divisions are — and losing control of a sea zone can strand an entire invasion force mid-crossing, sunk before it ever reaches the beach. Air power works as a resource allocated across regions rather than individually piloted, contesting air superiority that then modifies everything happening on the ground beneath it: a front held under enemy air superiority bleeds organisation and supply far faster than one held under contested or friendly skies. Both systems are easy to under-invest in because their payoff is indirect, showing up as a smoother land campaign rather than a visible battle won, and that indirection is exactly why players who master them have such a durable edge over players who don’t.

The ancestor and the alternate-history culture it built

HOI4’s lineage runs directly through Paradox’s own earlier Hearts of Iron games, which spent three prior entries and over a decade simplifying an operational-wargame tradition — hex-based, counter-pushing simulations of the same war — into something a single player could run in real time without a staff of assistants. What HOI4 added on top of that inheritance was a focus tree flexible enough to support genuine alternate history: campaigns where a non-aligned Poland industrialises fast enough to hold the 1939 border, or a democratic Germany avoids the war altogether, have become as central to the game’s culture as the historical path, in a way Hearts of Iron’s cousin titles at Paradox only later adopted through their own mission-tree systems. Total War: Warhammer III shows the same appetite for a flexible campaign layer solved through an entirely different, more fantastical lens, and the contrast says something about how differently “grand strategy” gets defined once history stops being a constraint.

The expansion model and an uneven depth

Hearts of Iron IV has been sustained by roughly a decade of paid expansions — Together for Victory in 2017, Waking the Tiger and Man the Guns through 2018 and 2019, La Résistance and Battle for the Bosporus in 2020, No Step Back in 2022, and Arms Against Tyranny and By Blood Alone across 2022 and 2023 — each one rebuilding a specific mechanic or nation cluster rather than simply adding cosmetic content. Man the Guns reworked naval design and gave Britain and the Commonwealth nations genuinely distinct focus trees; No Step Back rebuilt the Soviet Union’s internal politics and added a new supply-and-manpower model still felt across the whole game; La Résistance turned occupation and partisan resistance into an actual system rather than a background nuisance. The trade-off is real: a nation covered by a dedicated expansion plays with a depth and specificity the base game never approaches, while a country left untouched by any DLC — plenty of them, even after this many releases — still runs on the comparatively thin focus tree it shipped with in 2016. Playing Yugoslavia or Thailand today is a noticeably flatter experience than playing Germany or the Soviet Union, and that unevenness is a direct, structural consequence of how the expansion model has chosen to spend its development time, one country at a time, rather than a flaw in the base game’s design.

The multiplayer meta most players never see

Underneath the singleplayer experience most reviews describe, HOI4 supports a small but persistent competitive multiplayer scene, organised around private lobbies and league-style tournaments rather than public matchmaking. That scene has, over the years, forced its own house rules onto the base game — banning or restricting specific naval-invasion timings, garrison exploits and air-stacking tactics that a patient enough player can use to break the AI-tuned balance wide open — because a system this interlocking, built for a single human playing against scripted opponents, inevitably contains exploits that only surface once two equally skilled humans are trying to beat each other rather than the computer. Watching a competitive HOI4 match is a genuinely different experience from a typical singleplayer campaign: the pacing is faster, the opening moves more rehearsed, and the whole game reveals a layer of min-maxed optimisation that the AI opponent in a normal campaign would never demand of a player.

Spoilers below

The specific ways a Hearts of Iron IV campaign actually collapses, and the mechanics that produce the game’s stranger emergent stories, live here.

Lend-lease and volunteer systems let a nation stay technically neutral while materially deciding the outcome of someone else’s war, sending divisions of “volunteers” or shipping equipment to an ally without a formal declaration — a fairly direct model of the pre-Pearl-Harbor United States, and one of the few mechanics in the game that rewards a player for staying out of a war they could easily win, because the equipment and experience gained from volunteer deployments compounds quietly while the rest of the world burns its own industrial base down.

Puppet states and peace conferences are where HOI4’s political layer produces its strangest results. Winning a war doesn’t automatically redraw the map the way it would in a more abstracted strategy game; a peace conference splits the spoils among the winning coalition according to war participation score, which means a minor ally who did comparatively little fighting can still walk away with a disproportionate slice of the defeated nation’s territory if the score lands their way, and a major contributor can be shut out by a coalition partner with better timing. It is the single mechanic most likely to produce a genuinely surprising post-war map, and the community’s screenshots of a campaign’s final borders are almost always stranger than the historical ones.

The modding scene deserves its own mention, because it has quietly become as important to HOI4’s identity as anything Paradox has shipped directly. Total conversion mods swap the entire 1936 to 1948 historical scenario for invented timelines — a world where the First World War never ended, or one branching from a different outcome to an earlier conflict — built on the same focus-tree and division-designer tools the base game exposes to players. The scale of some of these projects, built by volunteer teams over years and downloaded by audiences that rival the base game’s own player count, says something about how well-suited HOI4’s systems are to reuse: a division designer and a focus tree are general-purpose enough to model almost any twentieth-century conflict, real or invented, and the modding community has spent the better part of a decade proving it.

The verdict, seven years on: Hearts of Iron IV is the most ambitious model in Paradox’s catalogue, and the ambition mostly holds, with the caveat that the AI’s grip on its own supply and production systems has always lagged behind the player’s, which can make the late game feel like managing an opponent who has stopped fully understanding its own economy. Anyone who wants the same era resolved through diplomacy and nation-building rather than division templates should try Europa Universalis IV’s mission-tree successor state next; anyone who wants the operational-scale tension without the historical stakes should sit down with Total War: Warhammer III instead, and see how much of HOI4’s appeal survives the swap from real history to invented war.