"Resistance movement" is a catch-all term for a group or collection of groups that seeks to oppose, either through violent or non-violent means, some entity the resistance deems oppressive. It may be a group that opposes an invading or occupying force, or it may oppose its own sovereign government. In popular culture, the single word Resistance, with a capital R, is often used to mean specifically the French struggle against Nazi occupation during World War II.
In Forthweg, the resistance battled Algarvian forces in Eoforwic before being crushed by Unkerlanters who feared the resistance would make Unkerlanter occupation of Forthweg difficult after the war ended.
In Grelz, an anti-Algarvian resistance attempted to sabotage Algarvian armies. It frequently battled anti-Swemmel Grelzers who supported the Algarvian-backed puppet Kingdom of Grelz under false King Raniero.
When it became clear that the United States was losing the Final War, the government set aside certain military personnel in hideouts such as the Nantahala Redoubt, to put up effective resistance against the Draka.
Resistance movements took place throughout the world in both the Great War and the Second Great War. These were movements in which members of assorted disaffected classes of the societies of combatant nations, usually with as much support as possible from their nations' enemies, rebelled against the countries they hated. Some of these campaigns were conventional military campaigns, some were paramilitary, and some were terroristic in nature.
In the Second Great War, resistance movements were much more common. The Mormons once again rose up against the US with Confederate support, as did the Canadians, with British support. The Mormons started using the tactics of "people bombing"- that is, attaching explosive to oneself, mingling with the enemy inconspicuously, and detonating the explosive- and car bombing. The US supported black people guerrillas in the CS as well as an uprising against Freedom Party rule in Cuba beginning in 1943. In Europe, the Central Powers backed resistance movements by the Irish (whose country had been reconquered by the British) and the Finns, the Jews, and some Poles against Russia. The Entente supported other, anti-German Poles, Ukrainians, and various ethnic minorities in the ethnically diverse Austro-Hungarian Empire.
When the Raceinvaded Earth and occupied numerous countries, resistance movements sprang up in occupied territories. In China, the resistance united former enemies from within China, and even included some Japanese ex-occupiers. The Polish resistance included Jews who had fought for the Race until realizing that the "Lizards" intended to use them as a catspaw against the rest of humanity.