Darkest Hour differs from Arsenal of Democracy (2010), the other modded product of Hearts of Iron II (2005) in that it expands the total number of provinces and functions as a fully moddable core. While improving Hearts of Iron II, Darkest Hour also includes a number of historical scenarios ranging from Operation Overlord to The Great War (World War I). Although it lacks the Indo-Pakistani War of 1947 or the 1948 Arab-Israeli War offered by Arsenal of Democracy, Darkest Hour provides one of the greatest user modifications ever made -- Kaiserreich.
Available for download on the Paradox forums, the Kaiserreich mod follows the "Legacy of the Weltkrieg." In this scenario, Wilhelm II secured victory in the First World War causing the Allied Powers to fall into disarray, and rendering the Nazis and their rise to power null. Russia, the United Kingdom, and the United States all experience turmoil and strife offering the player a large number of decisions -- the UK turns socialist, the US becomes fragmented... Mittelafrika is ruled by Hermann Göring... it's all very engaging.
During my playthrough as Romania in both Arsenal of Democracy and Darkest Hour, Darkest Hour kept most historically accurate during the 1936 Road to War campaign. However it should be noted that Slovakia is formed almost always in Arsenal of Democracy.
I highly recommend Darkest Hour to the armchair general sort of strategy fan. Its successor, Hearts of Iron III, is unfortunately bogged down with unnecessary complexity with an Order of Battle organizational structure that leaves users micromanaging at all times.