Allen was born in rural Connecticut and had a frontier upbringing, but he also received an education that included some philosophical teachings. In the late 1760s, he became interested in the New Hampshire Grants, buying land there and becoming embroiled in the legal disputes surrounding the territory. Legal setbacks led to the formation of the Green Mountain Boys, whom Allen led in a campaign of intimidation and property destruction to drive New York settlers from the Grants. He and the Green Mountain Boys seized the initiative early in the Revolutionary War and captured Fort Ticonderoga in May 1775. In September 1775, he led a failed attempt on Montreal which resulted in his capture by British authorities. He was imprisoned aboard Royal Navy ships, then paroled in New York City, and finally released in a prisoner exchange in 1778.