La Résistance is the expansion that introduced espionage as a serious mechanic in Hearts of Iron IV, on top of overhauling France and expanding Spain. Years later, it's still one of the ones that most changes how a long occupation plays out.
Nations and Content
It completely overhauls France's focus tree and significantly expands Spain's — the Spanish Civil War gained all the depth it has today here, with separate focuses for the Nationalist side and the Republic. It also adds content for Portugal.
New Mechanics
The big addition is the espionage layer: you can infiltrate agents into other countries, steal technology, sabotage their industry or army, and decrypt their radio codes to see part of their war plan. On the occupation side, it adds support for resistance movements in occupied territory and the option to install collaborationist governments instead of directly annexing territory.
Is It Worth Buying?
If you're interested in playing Spain or France in depth, it's practically mandatory — a big part of those focus trees' identity comes from here. Even if you're not going to play either, espionage and collaborationist governments affect any long game with occupied territory, making it one of the major expansions with the most cross-cutting impact.