- 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 Chinas official history of a United Front against the Japanese).
- 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 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.
Trivia / Hearts of Iron