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.
ISA was created to provide defense for Earth colonies to prevent UCN from spreading its forces too thin. To prevent any colony from ever rebelling against the UCN, the ISA forces are kept intentionally weaker than the UCA.