Hearts of Iron IV has been piling up expansions since 2016, and by now the list intimidates anyone starting out. This guide separates the major expansions (they change mechanics for the whole game) from the smaller packs (they add specific countries), so you know where your money is going.

Major Expansions

These are the ones that truly change how the game plays, not just which countries have a focus tree:

For Spain specifically: La Résistance is the expansion that gave it real depth, with an expanded civil war and separate focuses for the Nationalist side and the Republic. If you want to play Spain in depth, it's priority number one.
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Country or Theater Packs

These are cheaper and don't bring big new mechanics, just focus trees for specific countries:

These last four, together with the cosmetic Warships of the Pacific, make up Expansion Pass 2, focused entirely on the Pacific theater.

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

If you're only going to buy one major expansion, make it the one covering the country or region you're most interested in playing — there's no point paying for advanced naval mechanics if your plan is to play Spain or Germany on land. As a general rule: La Résistance or No Step Back give you the most mileage if you're playing in Europe, and the small country packs are only worth it if that specific nation appeals to you.