In the 1975 case New Hampshire v. Maine, the U.S. Supreme Court was tasked with resolving a border dispute between the two states over their shared boundary in Portsmouth Harbor and along the Piscataqua River. The disagreement arose from differing interpretations of a decree issued by King Charles I in 1622 that established boundaries for several colonies, including what would later become New Hampshire and Maine. In its decision, the court applied principles of estoppel - a legal doctrine preventing someone from denying facts that have already been settled - to rule against New Hampshire's claim to sovereignty over part of Portsmouth Harbor and portions of the river up to "low-water mark" on Maine’s shore. The court found that previous agreements between both states had recognized this area as belonging to Maine, thereby barring New Hampshire from asserting otherwise now.

In the dissenting opinion for New Hampshire v. Maine, 1975, Justice William O. Douglas argued that the court should not have used judicial estoppel to prevent New Hampshire from claiming territory it had previously ceded to Maine in an earlier agreement. He believed this was a matter of state sovereignty and that states should be allowed to change their minds about such agreements if they believe it is in their best interest. Furthermore, he disagreed with the majority's interpretation of historical documents and maps used as evidence in determining the boundary line between these two states. He felt there were ambiguities and inconsistencies within these sources which made them unreliable for establishing definitive boundaries.