Before joining the union what state was an independent republic from 1777 to 1791?

The Hampshire Grants was disputed territory between New York and New Hampshire. Although neither of those states was ever fully able to exert control over the grants, it is inaccurate to say the territory was an independent republic. It had no well established government, it carried on no foreign policy and it's residents were citizens of the United States. It was finally organized and admitted to the Union as the state of Vermont.