Hearts of Iron IV has been piling up DLC since 2016, and by now the Steam store page scares off anyone new to the game. This guide sorts the wheat from the chaff: the major expansions that change mechanics across the whole game, versus the smaller packs that just add specific countries — so you know exactly where your money is going before you spend it.
Major Expansions
These are the ones that truly change how the game plays, not just which countries have a focus tree:
La Résistance (2020) — 
France (overhauled), 
Spain (expanded), 
Portugal. Adds espionage, resistance movement support, collaborationist governments and code-breaking.
No Step Back (2021) — 
USSR and 
Poland overhauled, Baltic states. Adds an officer corps, army spirits, the tank designer and railway guns.
By Blood Alone (2022) — 
Italy overhauled, 
Ethiopia, Switzerland. Adds the plane designer, peace conference options, unit medals and embargoes.
Arms Against Tyranny (2023) — 
Finland, 
Sweden, Norway, Denmark, Iceland. Adds Military Industrial Organizations (MIO) and the international arms market.
Götterdämmerung (2024) — 
Germany rewritten, 
Austria, 
Hungary, Belgium, Belgian Congo. Adds special projects (nuclear weapons, rockets, advanced tanks) and the raid system.
No Compromise, No Surrender (2025) — 
Japan, 
Nationalist China, 
Communist China, Philippines. Expands the faction system, adds new doctrines and carrier naval mechanics.
Country or Theater Packs
These are cheaper and, except for Thunder at Our Gates, don't bring big new mechanics — just focus trees for specific countries:
Battle for the Bosporus (2020) — 
Bulgaria, 
Greece, 
Turkey.
Trial of Allegiance (2024) — 
Brazil, Argentina, Chile, Paraguay, Uruguay.
Graveyard of Empires (2025) — 
India, 
Iran, Iraq, 
Afghanistan.
Peace for Our Time (2026) — 
Czechoslovakia, with an alternate resistance scenario against Germany at Munich.
Thunder at Our Gates (June 2026, the most recent) — 
Australia, 
Siam, 
Indonesia. Adds Army General Headquarters and ship captains.
Peace for Our Time and Thunder at Our Gates, together with No Compromise, No Surrender (above, among the major expansions) and the cosmetic Warships of the Pacific, make up Expansion Pass 2 — though, despite the pack's "Pacific" label, Peace for Our Time is set in Czechoslovakia, not that region.
What's Already Free
Through earlier updates, these have already been folded into the base game for free: 
Man the Guns (ship designer, naval treaties), 
Waking the Tiger (China's trees), 
Death or Dishonor (Hungary, 
Romania, Czechoslovakia, 
Yugoslavia), 
Together for Victory (
Canada, Australia, New Zealand, India, South Africa) and Poland: United and Ready.
Where to Start
My advice if you're only spending money on one major expansion: make it the one covering the country or region you actually want to play. There's no point paying for advanced naval mechanics if your plan is holding the trenches with Spain or Germany. As a general rule, La Résistance and No Step Back give you the most mileage if you're playing in Europe; the small country packs are only worth it once that specific nation appeals to you, not before.