Most people think Vermont was one of the 13 original colonies. It was not. Neither was Maine. At the time the area between New Hampshire and New York, both of which were in the original 13 colonies, was a dangerous wilderness. The Indians—often goaded on by the French Canadians who were not pleased that so many Englishmen were settling in the land just below them—had a nasty habit of coming down from the north and returning with the scalps of anyone daring enough to settle in what is now Vermont (and upstate New York north of Albany.) But, once the French and Indian war was settled between the France and England in the decade just preceding the American Revolution, a torrent of settlers from crowded Connecticut and Massachusetts, both of which were also part of the 13 colonies, came northward looking for a farm of their own.
Benning Wentworth, the Governor of next door New Hampshire, sensing a way to make some extra cash, granted this wilderness land to the settlers. The problem was it was not his land to grant. It technically belonged to New York. But New York officials were preoccupied with other matters to do much about it until much of the land was already sold and settled in what was known as “the New Hampshire Grants.” When New York finally realized what was happening it tried to take action. New York tried a number of things—it granted the same land to New Yorkers, tried to get those already settled on the disputed land to pay them for it, and even tried to kick the trespassers off. None of it worked. The settlers revolted. “Yorkers” who ventured into Vermont to survey or claim the land were treated badly and chased off. That is where the “Green Mountain Boys” came from.
The non-state of Vermont played an important role in the American Revolution and would have welcomed the chance to join the newly formed country. But the Continental Congress was unwilling to upset such a powerful state as New York by making Vermont a state. So Vermont lingered on in limbo through the entire American Revolution and beyond.
After the American Revolution was over a compromise was reached with New York and Vermont was made the 14th state on March 4, 1791.
At the time, it was important to keep the balance of power between the northern and southern states. At the same time, Kentucky was trying to break away from Virginia, of which Kentucky had been a part of. So a year after Vermont was admitted, Kentucky was made the 15th state in June 1792.
This north-south-north bounce continued as the next ten states were admitted over the next 30 years. A northern state was admitted, then a southern state.
Maine was not admitted until 1820 and was the 23rd state. Before that it was part of Massachusetts.