Westmoreland County, Connecticut was a county established by the State of Connecticut in the present-day area of Wyoming Valley, in northeastern Pennsylvania. Both colonies claimed this territory until after the American Revolutionary War, when Connecticut ceded it to Pennsylvania in 1784 as part of a national resolution of outstanding claims. This territory now forms the northeastern corner of Pennsylvania.
It briefly seceded to become the State of Westmoreland. It has no relationship to the current Westmoreland County, Pennsylvania, which is located on the western and opposite side of the state.
References
Westmoreland County, Connecticut Wikipedia(Text) CC BY-SA