When the UCN was formed, the UCN Defense Force was its military. However, in the midst of expanding colonization and reports concerning allegations of corruption, military inefficiency, and a sluggish bureaucracy, the UCN Defense Force was officially dissolved in the early 2100s in favor of a more efficient service, with the United Colonial Army being formed thereafter. The UCA Navy is the first service branch formed.
Unlike the ISA which relies on local funds from their respective systems to function, the UCA receives funding directly from the UCN. This provides it with more resources and power than any typical ISA branch. The UCA and its Navy have weapons, training and technology far beyond the level of individual ISA commands.
The UCA Navy is a force not to be challenged. During the First Extrasolar War, the UCA Navy destroyed the entire Helghan Administration's fleets and their orbital facilities in a matter of hours without losing a single vessel.
The UCA Navy also provided assistance to the ISA troops during the Invasion of Vekta by Helghast forces in the Second Extrasolar War, but arrived after the initial invasion. They are the most powerful force in the Killzone Universe.