1783-1853
The 1783 Treaty of Paris with Great Britain defined the original borders of the United States. Due to ambiguities in the treaty, the ownership of Machias Seal Island and North Rock remains disputed between the U.S. and Canada; other original territorial ambiguities (including the Northeastern Boundary Dispute and the disputed Indian Stream territory) were resolved by the Webster-Ashburton Treaty in 1842.
Read more about this topic: United States Territorial Acquisitions