The Ohio National Guard can be traced back to the initial settlement at Marietta, Ohio, in July 1788. Rooted in the English and early colonial tradition of citizen-soldiers providing local protection and law enforcement, these Revolutionary War veterans and their families quickly organized into local militia units. Reflecting the provisions of the U.S. Constitution establishing the need for "a well regulated militia being necessary for the security of a free state," the federal government passed the Militia Act of 1792 which required all able bodied men ages 18-45 to serve in their local militia units and provide their own weapons and equipment. It further authorized the Governor of each state to appoint an Adjutant General to enact the orders of the Governor and to supervise unit training and organization. Reflecting the founding fathers' distrust of a large standing army, it strictly limited the ability of the militia to serve outside of their state borders and placed effective control with the Governor rather than the federal government.
As settlement spread across the Ohio Territory, a confederation of Indian tribes with British backing engaged in a campaign of raids and depredations upon the scattered settlements. The disastrous campaigns led by Generals Harmer and St. Clair only intensified Indian resistance to white migration and threatened the existence of the existing Ohio settlements. Not until the decisive victory of Gen. "Mad" Anthony Wayne at Fallen Timbers, outside of present day Toledo and the subsequent Treaty of Greenville was the combined British-Indian military threat to Ohio removed. While only limited numbers of Ohio militia played a part in these campaigns, the local militia units formed an important bulwark defending their local communities against potential attack.