Vermont was admitted as the fourteenth state on this day in 1791. The first state to join that had not been part of the 13 colonies that had formed the United States, Vermont's history had still been inextricably tied up in the Revolutionary War.
The main impetus for its break from the British Crown came in 1764, when the king issued both New York and New Hampshire landowners grants to tracts of land in Vermont that overlapped. The conflict turned violent, and the Green Mountain Boys militia led by Ethan Allen pushed New Yorkers out through force by 1775.
When the American Revolution broke out, separatists in Vermont joined in. The 13 colonies, though, saw it as part of New York. Vermonters disagreed and, unable to join the fledgling United States, opted to declare themselves a republic as well in 1777. In 1790, after much of the revolutionary dust had settled, New York relented and recognized Vermont. Once the two entities agreed on a clear border, a proposal to join the U.S. sped through the tiny republic's legislature. Three months later, it became the 14th state in the Union.
You can learn more about Vermont's time as an independent republic and admission to the U.S. at the Encyclopedia Britannica.