Hearts of Iron

Hearts of Iron was founded by Swedish studio Paradox Interactive in 2002 as a grand-strategy simulation of the Second World War, where the player runs one of the warring nations across industrial production, military research, diplomacy, and command of armies right on the front line. The series set itself apart from other war strategy games by tying economic planning to real-time command over a map of the entire world, so a decision about how much steel goes into tanks versus ships had a direct effect on the battlefield months later, putting the player in charge of the war as a whole rather than any single battle.

Hearts of Iron II in 2005 cleaned up the controls and expanded diplomacy and so-called alternate-history options, letting the player's choices diverge from the real course of the conflict, and it became a long-standing reference point for genre fans as a result. Hearts of Iron III in 2009 then deepened the system considerably, with more elaborate logistics, province-level micromanagement, and more detailed army organisation, pleasing players after maximum complexity while noticeably raising the barrier for newcomers and stretching out the time needed to learn the basics.

The series remains one of the few major strategy franchises that keeps developing mainly through large expansions deepening individual mechanics rather than jumps between whole new generations, and Paradox has continued that approach in later entries, which at their core still build on the foundation laid by the first three games.