Undeath & Recursion
The Chitin Sovereignty is an insectoid civilization that rose from a single teeming homeworld and never stopped multiplying. The Sovereignty's castes handle trade, science, and war the way a hive divides labor, and the population crisis that strains the other alliances is, for the Chitin, an existential clock. Broods hatch faster than worlds can hold them. The Sovereignty does not see the new system as territory to negotiate over. It sees room, and it will take it.
The Legion is the Sovereignty's militant caste, and the reason its enemies fear long wars. The Legion reanimates its fallen, returning dead soldiers to the line again and again until the enemy runs out of living ones. In a galaxy where every alliance is drowning in population, the Legion's arithmetic is the coldest of all: it never truly loses troops, only ammunition. The contest for the new system is exactly the kind of grinding attrition war the Legion was bred for, and it has whole armies of the dead waiting in its holds.