The Diggers and Levellers emigrate from Britain to somewhere in North America after the Restoration, and establish a separate colony of their own in a small area that nobody else wants too much. This comunity doesn't play an active role in the American Revolution (they're pacifists...), but Britain doesn't consider trying to hold on to that minor enclave worthwhile when it's giving up the surrounding lands and consequently recognises its independence too. The people there decide against joining the USA, for ideological reasons, and existing agreements with the other colonies -- plus maybe the wording of the treaty that recognised ALL of the colonies' independence, which the others don't want to be seen breaking too blatantly -- protect it from annexation. The separate existence of this state becomes something that's simply taken for granted...