I don't get the impression that Earth has a military force other than Starfleet. And I get the impression most other planets have only small defense forces. The only time we really hear about a Federation planet which has its own defense force is Vulcan in Unification, and you don't get the impression it can really defend itself against an invasion.
Also when Bajor was going to join the Federation it was stated that the military was going to be absorbed into Starfleet.
My impression of the Earth government is that it's more an internal governance structure and doesn't have power over Starfleet military issues.