What is the only U.S. state to declare war on another country?

According to history, the only U.S. state to declare war on another country is Maine. The state of Maine made its declaration concerning the Aroostook War (sometimes called the Pork and Beans War). It was a military and civilian-involved confrontation in 1838–1839 between the United States and the United Kingdom over the international boundary between the British colony of New Brunswick and the U.S. state of Maine.

The Aroostook War was a bloodless conflict over the disputed boundary between the U.S. state of Maine and the British Canadian province of New Brunswick. The peace treaty of 1783 ending the American Revolution had left unclear the location of a supposed “highlands” or watershed, dividing the two areas. Negotiators from Britain and the United States in subsequent years failed to come to an agreement, and the matter was referred to the king of the Netherlands, who in 1831 rendered a decision that the citizens of Maine objected to strenuously, forcing the U.S. Senate to reject it.

Top-level diplomats from the U.S. and Britain met in Washington and forged a peaceful compromise, the Webster–Ashburton Treaty in 1842. It fixed the permanent border. The term "war" was rhetorical; local militia units were called out but never engaged in significant combat. The event has been best described as an international incident.