For many years, the nearby colonies, especially New Hampshire and New York, disputed control of the area (then called the New Hampshire Grants). Settlers who held land titles granted by these colonies were opposed by the Green Mountain Boys militia, which eventually prevailed in creating an independent state, the Vermont Republic.
Many have heard of the "Thirteen Colonies" though there is evidence that there may have been only 12 at some point. Vermont could be regarded to be the 14th. Vermont is distinct in that its declaration of independence came in 1777 rather than in 1776.
Vermont Republic joined the United States of America in 1791 as an originally-sovereign state like the original twelve-or-so states of America. Vermont Republic's independence unlike the other united states of America resulted from military organization called the Green Mountain Boys (headed by Ethan Allen) in what appears to have been a territory of New Hampshire having prevailed against New Hampshire and New York! The Vermont Republic after fourteen years eventually joined the United States of America in in 1791. The sovereignty of Vermont was upheld in an 1815 court case (see scans below).
On March 20, 1764, King George III established the boundary between New Hampshire and New York along the west bank of the Connecticut River, north of Massachusetts, and south of 45 degrees north latitude. In 1770, Ethan Allen, his brothers Ira and Levi, and Seth Warner recruited an informal militia, known as the Green Mountain Boys, to protect the interests of the original New Hampshire settlers against the new migrants from New York.
Aside from the Thirteen Colonies, Vermont is one of only four U.S. states (along with Texas, Hawaii, and California) to have been a sovereign state in its past.
(Please move to Right of Self Determination forum.)