The Cabal Corps (Turii: Sarxa Usdor) serves as the Turian Army's biotic program. In contrast to the Systems Alliance's SENTINEL and VANGUARD programs, it is comprised nearly completely of self-contained, all-biotic units which are only rarely integrated with general infantry forces. Instead, Cabal units receive the full curriculum of Turian Army combat, technical, and officer training in addition to extensive instruction in combat biotics. The Cabal Corps includes many of the specialty combat units employed elsewhere in the Turian Army including naval infantry, special forces, covert operations and hazardous-environment teams; technical, medical, and logistical support is provided by non-biotic personnel from elsewhere in the turian military, both due to the relative rarity of turian biotics necessitating that as many as possible be trained in combat, and as a measure to prevent the highly insular organization from acting without Hierarchy oversight.
The Corps is additionally unique in that biotic turians are inducted shortly after birth and raised communally. Close to ninety-five percent of turian biotics have at least one parent in the Cabal Corps (while the practice is heavily discouraged by Hierarchy medical authorities, many Cabalists deliberately consume or otherwise expose themselves to trace amounts of Element Zero to increase the likelihood of biotic abilities in their children), but many non-biotic turians continue to view the Corps as 'child-snatchers' as a result. This, coupled with the frequent occurrence of cosmetic birth defects (which are corrected at an early age through surgery or gene therapy in the rest of the Hierarchy), the Corps' highly insular nature, and occasional reports of 'cult-like' practices among Cabal members, has made it somewhat unpopular among other turian military units. A particularly intense rivalry exists between Cabalists and the generally highly-regarded Recon, which shares a similar mission profile and often competes for funding and support- a common proverb circulated in Turian Army combat units claims "Scouts work for a living. Cabalists cheat." Somewhat less well-known is the Corps' rejoinder, "A fair fight is a poor fight. Cabalists cheat to win."
In light of the disciplinary issues arising from their social estrangement (Cabalists are three times more likely to join separatist organizations than non-biotic turians, and five times more likely to abandon their Hierarchy citizenship) High Command has made periodic attempts to dissolve, restructure, or integrate the Corps into other Army units, always unsuccessfully.
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Author's Notes:
I typically don't directly borrow ideas directly from the other people who I consulted with in writing PD, but Archangel1207's bit about Cabalists deliberately exposing themselves to Element Zero to have biotic kids I thought was too neat to pass up.
Astute readers may have mentioned that only a Turian Army and Turian Navy have been mentioned, while the Alliance has only been shown to have a Navy and a Marine Corps. This is indeed correct, and there is no such thing as the Alliance Army or Turian Marines. This helps keep the two forces easily identifiable and I always found the existence of a dedicated naval infantry force completely separate from all the other infantry forces to be a bit strange (for the longest time I've wanted to go over standard military organization to make it more modular), so all (or nearly all?) of the Citadel powers have their naval infantry integrated into the same branch as their main army forces.
For the turians that's the Turian Army, although they may consider specialties like the medical corps, engineering, etc. to be branches in and of themselves on par with the Army and Navy, I'm not sure yet. Given how much public service and general administrative stuff in the Hierarchy is military-run, however, I think it's likely that the military units responsible have a greater prominence.
For the humans their single ground-combat branch is called a Marine Corps for the simple reason that Marines appear in the games. I am actually sort of wondering if this may have come from the early Alliance trying to hold on to the traditional distinction and deploying only Marines to its outermost colonies, resulting in the SAMC gradually taking on more and more of the roles of a traditional army and whatever dedicated air forces survived the transition into spaceflight while the 'official' branches that were supposed to be doing that stuff just kind of withered into irrelevance back on Earth. I like the sort of ironic, self-defeating quality of this in that it was very probably done for the benefit of the people who had Strong Opinions on the uniqueness and division of the various different military branches, and wound up compressing everything into a single disorganized jumble. I don't know if the SAMC is still operationally subordinate to the Navy, but we see a lot more Admirals than Generals in the games so I think it probably is.
If I have the time I might actually draw up a full description of how the top-level government/military structures of the various Citadel Powers are configured; not sure how many other people here share my fascination with org charts, but if I get a positive response to these entries I'll go ahead with it.