plawolf
Lieutenant General
The SS is the closest comparison. Loaded though it may be.
The USMC began much like any other late 18th early 19th century Marine corps as naval sharp shooters and raiders. But by the 20th century expanded into a number of roles including amphibious assault and close air support.
However the Marines remain Naval assets at heart, always returning to the sea. There amphibious assault roles were a natural extension from naval raiding, and even today a significant amount of the mission logistics and support of the USMC is rooted in the Navy. The medical, intelligence and Religious support system is Navy not Marines. It was a natural expansion to meet the modern world where ship to ship combat was at beyond rifle range.
I thought about trying a comparison to the PLA and how a infantry arm expanded into multiple services and subservices but it was rooted to the same mission and so that expansion was a natural progression. They were given the mission by the leadership, "Fight for the Party, Defend the PRC" so when the PLA hit the sea they created the PLAN, when they hit the Air PLAAF and so on and so forth. But they were still obeying the mission mandate.
By contrast the IRGC and SS both began as internal agencies the IRGC the enforcement of Islamic ideals at home, the SS the top leadership's personal security. But from those missions they expanded well beyond and became external forces.
It would be like the NYPD launching a full space program. How does the Original mandate IE Protect and serve NYC suddenly include Mars?
I really don't see it. The lines you are drawing seem kinda deliberate rather than categorical.
The USMC is hardly limited to only, or even mainly amphibious assault. You have to go back to the Korean war for a real life example where their amphibious assault capabilities were needed to take a defended beachhead, and even then it wasn't really strictly needed since the beaches were so lightly defended.
In all recent wars, the USMC has been employed as expeditionary elites, and spent far more time on land than they did out at sea. Hell, most of the time they didn't even deploy into war zones from ships, and instead entered combat from land bases and staging zones alongside the regular army.
You originally used the SS to highlight how the IRGC expanded from a specialist single role organisation to encompass entirely new fields and disciplines, Like naval and missile branches etc.
For that illustration, the USMC would be a much more accurate companions, since it also grew from a single niche role to add entirely new roles and fields to their force organisation, with all the hardware and support infrastructure that goes with that.
In contrast, I am not aware of the SS developing dedicated air or naval capabilities. The armed branch of the SS remained a ground operations oriented combat force.
Had you originally started by talking about the political aspect of how the IRGC is acting as trusted enforcers and guardians of the government, then yes, the SS comparison would be more valid.
But in terms of organisational expansion and 'mission creep', they are not much like the SS, who showed little interest in adding naval or air assets to their inventory and expand themselves into those fields and disciplines, and instead more akin to the USMC in their desire to acquire assets and capabilities beyond their original remit and force structure.