Diplomacy is a blunt instrument to pull someone closer to your side, or push them into war. Your population exists not to be taxed, but to be given a gun and a mission.
Trade is highly abstracted and focused on specific materials.
You acquire new weapons through factories. To this end, Hearts of Iron has a much greater emphasis on military matters than political or economic matters. But Hearts of Iron is still a game about war – preparing for war, researching war related technologies, developing attack plans and then keeping that war machine going until the issue is settled. Maybe Germany goes after the Soviet Union first, and maybe Italy decides to go its own way in 1938. Of course, the World War II you encounter in a game will bear only a passing resemblance to the one we know from history. You are not shackled to history, but the game is about World War II and World War II will happen. Unlike the broad open-ended stories that are available to you in Crusader Kings II or Europa Universalis IV, Hearts of Iron is a ruthlessly focused game. Hearts of Iron IV is one of Paradox Development Studio’s most popular titles, but also a bit of an outsider.