The Abenaki nation originated in what's now northern New England and the southern Canadian Maritimes. Although suggesting they've only one nation may be a bit of a misnomer, since "Abenaki" can refer to numerous groups from the same approximate geographic area and language family (Algonquin, since you asked).
Although the various Abenaki would join together during crisis events (like when they were under attack from other nations) for the most part they had no central authority. That's probably partly what caused the encroachment of British Colonists on their land. The British liked to have formal governments they could negotiate treaties with - it's just so much more civilised and sometimes there are canapés - so from the Colonists' perspective, no central government meant that the Abenaki had no claim on the land. Of course, they could have found out otherwise, but that would have taken work, and been complicated, and - perhaps most importantly - the Abenaki might have said 'no'.
While the Abenaki tried to remain neutral during the French and Indian war, they were quite literally caught between the warring British and French colonies, so some conflict was inevitable. For the most part, they sided with the French, who were less responsible for driving them of their homeland. The lesser of two evil, I suppose you could say - though that's a sentence I never thought I'd say about the French.