Aroostook War an undeclared and bloodless conflict between Maine and New Brunswick, Canada, in 1839 that almost led to war between the United States and Great Britain. The dispute concerned 12,000 square miles (31,000 km2) of rich timberland in the Aroostook River valley and resulted from the failure of the United States and Great Britain to clearly define the northeastern boundary of what is now Maine in the Treaty of Versailles ending the American Revolution.

Following its admission as a state in 1820, Maine made grants of land along the Aroostook River, disregarding Canadian claims. During the winter of 1838–39, lumberjacks from New Brunswick entered the disputed area and began lumbering operations. After Canadians captured a Maine expedition sent to expel them, both Maine and New Brunswick called out their militia. Full-scale conflict was averted when General Winfield Scott, representing President Van Buren, negotiated a temporary compromise between Maine and New Brunswick. The issue was settled by the Webster-Ashburton Treaty of 1842.