The Toledo War was a border dispute between the State of Ohio and the Michigan Territory during the first several decades of the nineteenth century.

The Northwest Ordinance of 1787 established the northern boundary of Ohio as "an east west line drawn through the southerly bend of extreme of Lake Michigan." The United States Congress restated this line as Ohio's northern boundary in the Enabling Act of 1802, the legislation that led Ohio to become a state. The Congress also used the same language when it established the Michigan Territory in 1805. The Congress incorrectly believed that the line would intersect Lake Erie north of the Maumee River's mouth. During the Ohio Constitutional Convention of 1802, delegates received word from a fur trapper that the line would actually fall south of the Maumee River. The convention delegates stipulated in the Ohio Constitution of 1803 that Ohio's northern boundary must include the Maumee River's mouth. The United States Congress accepted Ohio's constitution, but it never formally acted on the convention's boundary proviso. The failure of Congress to act led to thirty years of conflict between Ohio and the Michigan Territory.

In 1812, the Congress authorized a survey of the boundary and hoped to end the dispute. The War of 1812 prevented the survey from taking place. In 1817, Edward Tiffin, the surveyor-general of Ohio, ordered William Harris to survey Ohio's northern border on the basis of the boundary established in the Constitution of 1803. Lewis Cass, the governor of the Michigan Territory, authorized his own survey based on the Enabling Act of 1802's boundary, which resulted in the Fulton Line. Both sides refused to compromise in the dispute.

By 1833, Michigan was moving towards statehood. Ohioans feared that Michigan would secure the disputed land if the issue was not settled before Michigan became a state. In 1833, the United States Senate sided with Ohio, but the House of Representatives refused to endorse the Senate's view. The Governor of the Michigan Territory, Stevens Mason, proposed the formation of a commission to negotiate a solution. Governor Robert Lucas of Ohio refused Mason's proposal. In 1835, the Ohio legislature formed Lucas County out of this disputed territory. The Ohio Common Pleas Court also held a session in the region to further validate Ohio's claim.

As a result of Ohio's actions, Governor Mason called for the Michigan Territory's militia to stand ready to take the territory by force. Governor Lucas responded in kind, sending Ohio's militia into the disputed area. Mason asked President Andrew Jackson to intervene. Jackson sent two representatives to negotiate a peaceful resolution to the conflict, and both groups sent representatives to a commission to negotiate. Both Ohio and Michigan, however, made it clear that they were ready for violence. Governor Mason ordered the arrest of Ohio's commissioners.

The federal government's representatives suggested Ohio and Michigan jointly govern the territory until the United States Congress could decide the issue. The Ohio legislature agreed to this proposal but, at the same time, authorized 300,000 dollars to the Ohio militia to seize the territory by force if Michigan refused joint control. Mason refused to accept the federal government's proposal. The Ohio militia moved to the south bank of the Maumee River and faced the Michigan militia on the north side.

President Jackson removed Mason as governor of the Michigan Territory for refusing to cooperate with the federal government's representatives. John Horner replaced Mason. Horner worked with Lucas to reach an amicable conclusion to the dispute. Many people in Michigan despised the new governor for his actions. They hanged Horner in effigy and verbally assaulted him. On June 15, 1836, President Jackson ratified an agreement between the two governors. Ohio would receive the disputed area of roughly four hundred square miles of land and the Michigan Territory would finally become a state. Jackson also granted nine thousand square miles of the Upper Peninsula to Michigan. With Jackson's actions, the Toledo War finally came to an end.