- Banned in China: Because it has Tibet as an independent country with its own flag, which the PRC has banned since 1959, and so is Manchukuo, as well as having China divided among the various warlords (which contradicts China’s official history of a "United Front" against the Japanese). Somewhat Zig-zagged with IV, as while the base game is banned, you can still buy the expansions on Steam...which conveniently include the base game.
- Dummied Out: Hearts of Iron II has a scripted assassination event, where the US can kill a Japanese leader. Arsenal Of Democracy comments it out and makes it unusable, but it otherwise remains.
- Meaningful Release Date:
- Semper Fi, the first DLC for Hearts of Iron III, and Hearts of Iron IV were both released on the 6th of June - the exact same day as the Normandy Landings, and the day during the Battle of Midway when the Japanese cruiser Mikuma was sunk.
- Waking the Tiger (the third DLC for Hearts of Iron IV), as well as the accompanying free update, were released on 8th March (International Women's Day) of 2018. New content include a loading screen featuring two female Soviet snipers, a "women in the workforce" decision (if the country is at war, allowing women to replace men in factories allows to allocate more men as the available manpower for army recruitment), a "Rosie the Bren Gun Girl" and "Rosie the Riveter" events (the effect of taking the aforementioned "women in the workforce" decision, respectively of Canada and USA), Soong Mei-Ling (Chiang Kai-shek's wife) available as a Chinese minister, or Yoshiko Kawashima (a Chinese princess of Manchu origin who was raised in Japan and became a spy) available as a Manchukuo general.
- The release of the Allied Speeches pack for IV was released June 4th 2020, the 80th anniversary of Winston Churchill's "we shall fight on the beaches" speech.
- Similarly Named Works: The series shares its name with a song by Sabaton, from their 2014 album Heroes. The song was actually written as part of a promotional deal (some Sabaton fans at Paradox approached them after a show in Stockholm), and Paradox licensed "Hearts of Iron" and several other songs as part of soundtrack DLCs for HoI IV and Europa Universalis IV.
- Trolling Creator:
- A specific set of circumstances (some player influenced, some random) allow Victoria Louise to be crowned as Kaiserin of a restored Imperial Germany (and possibly the Holy Roman Empire). Her regal title? Victoria III. The Paradox fanbase always hoped that the next game announced would be a sequel to Victoria II, which happened in 2021..
- On November 1, 2017, podcat released a Developer Diary named "Airplanes and Lootboxes". On closer inspection, "lootboxes" was how podcat was referring to packets of airdropped supplies, but at least one person had a knee-jerk reaction thinking that this was the addition of Microtransactions to IV. He also followed up by saying he was not adding actual loot boxes to the game after viewing an article on the subject and provided a link. It was a Rickroll.
- What Could Have Been:
- No Step Back originally planned to have an off-map nuclear reactor for Poland to represent the work Polish scientists did on the Manhattan project. Due to this being seen as too immersion breaking, as well as many fans complaining, it was scrapped.
- Originally, Poland's monarchist path in IV would've only had 4.5 options (Friedrich Christiansen, Michael I, and Pavel Berdmondt-Avalov, with Anastasia Romanov and Wotjek being Secret Characters). However, the dev in charge of the path found time to implement a fourth option (Karl Albrecht von Habsburg).
- While the finished reworked Soviet Focus tree in No Step Back does not have a proper democratic pathnote , content shows that one would've been able to select Vsevolod Ivanovich as a Tsar, and he would've been able to lead a democratic Russia as a constitutional monarchy (in the final game, Vladimir III is the only choice).
Trivia / Hearts of Iron