Ohio Militia was formed in 1803. At that time, every state within the United States had its own militia. The militia existed to protect a state's residents from attack, whether it was from Indians, other countries, or internal revolts. Most states required able-bodied white men to participate in the militia. Age limits existed in all states, with most states requiring men between the ages of eighteen and forty-five to participate in the militia.
The Ohio Militia played an important role in the state's early history. Militiamen helped United States soldiers subdue Ohio's Indian people. During the War of 1812, the Ohio Militia helped defend the American Northwest from British invasion. Following this conflict, the Ohio Militia still existed, but it declined in importance. With the death of the Shawnee leader Tecumseh at the Battle of the Thames in 1813, most Ohioans no longer faced major threats from the Indians. The United States also remained at peace with other nations until the Mexican War began in 1846. During this prolonged era of peace, Ohio's state government increasingly reduced funding to the militia to set aside money for other programs.
With the outbreak of the American Civil War in April 1861, Ohio Governor William Dennison dispatched George McClellan and Jacob Cox to the state arsenal in Columbus to investigate the guns and other supplies that Ohio had on hand to help equip the state's militia units. The two men discovered a few crates of rusted smoothbore muskets, mildewed harness for horses, and some six-pound cannons that could not be fired. Despite the lack of equipment, Dennison encouraged Ohio communities to revive the militia system and form units that they would send to Columbus, the state capital. Dennison entrusted McClellan with command of these units, asking him to create a cohesive and professional force out of the volunteers. Most of these units formed the basis of Ohio Volunteer Infantry units, which the State of Ohio eventually provided to the United States government for the war against the South.
Following the Civil War, the Ohio Militia remained in existence, but it again experienced a decline. In 1903, the United States government implemented the Dick Act, which created the National Guard. State militia units now became National Guard units. The Ohio Militia became the Ohio National Guard. Before creation of the National Guard, state militia units only existed to protect their respective states.
States still oversaw the new National Guard units, which were primarily to serve in their own states. However, the federal government could now nationalize National Guard units and send them into other states or even other countries without the approval of the units' home states. Today, both Army National Guard and Air National Guard units exist in Ohio.
Despite the existence of the National Guard, Ohio still maintained its own militia force. The Dick Act prohibited states from retaining their own militia systems. Ohio was one of the states that chose not to follow the federal mandate and keep such a force. A principal reason for the continuation of the Ohio Militia was the desire to protect Lake Erie. There was no Naval National Guard. With Ohio having a water boundary with Canada, Ohio leaders felt the need to maintain a naval component to its militia force. In 2006, Ohio was one of only five states to with a naval force.
Following World War II, the Ohio government established the Ohio Defense Corps, which is now known as the Ohio Military Reserve. The Ohio Defense Corps was a continuation of the Ohio Militia. In 1961, Ohio implemented a law that made all of Ohio's men and women between seventeen and sixty-seven years of age eligible for duty in the Ohio Military Reserve. In 2006, the Ohio Military Reserve numbered just six hundred members. Its primary purpose was to expand quickly to assist Ohioans in case of natural disasters or enemy attack. The state government now relied primarily upon the Ohio National Guard for the state's defense, as well as to assist in natural disasters and other types of emergencies. However, if the Ohio National Guard was either unavailable or in need of assistance, the Ohio Military Reserve stood ready to support the Guard.
In the late twentieth century, private citizens in the state formed new types of military organizations. These units called themselves "militias" and were formed in a number other states as well as Ohio. These organizations said that they existed to defend individual states and the United States from attack. They also said that they would stand against federal and state government actions that the militia members believed to be unjust. Many militia members refused to abide by federal laws for various reasons. State and federal courts repeatedly ruled against many of the actions of these private militia units and their members.