The Flag of the Green Mountain Boys, predating the Vermont Republic.
The Republic of Vermont was born out of land disputes between the colonies of New York and New Hampshire. Both claimed the territory but New Hampshire’s governor started making land grants for colonists in 1749. New York started issuing land grants in 1765. Many of the grants were for the same land, leading to violence between rival claimants. The New Hampshire colonists organized a militia known as the Green Mountain Boys in 1770 to defend beneficiaries of the New Hampshire grants and run off colonists from New York. Sporadic conflicts continued until Vermont was established as an independent republic in 1777. It was not until 1791 that Vermont joined the United States as the 14th state.
The Green Mountain Boys also played a role in some of the early battles of the American War of Independence. Underground organizations like the Sons of Liberty had long been agitating, often through mob violence, against British soldiers and other authorities and arguing for American Independence. These conflicts led to the appointment of General Thomas Gage as the Royal governor of Massachusetts. When he ordered British forces to seize the military stores of Lexington and Concord, they were repelled by a number of local militia. This marked a turning point in the agitation for American independence as more colonists in New England and beyond began to rally around the besieged colony of Massachusetts.
On May 10, 1775, the Green Mountain Boys, along with some other colonial militiamen led by Benedict Arnold, conducted a surprise attack and successfully seized Fort Ticonderoga. They went on to assist with the seizures of Crown Point and Fort George, all British forts located in New York.
Sources:
Green Mountain Boys- Wikipedia
The Capture of Fort Ticonderoga- Fort Ticonderoga
The Vermont Republic- The History Guy
GMNF- The Original Vermonters- US Forest Service