Despite President Woodrow Wilson's design to remain neutral, the United States declared war on Germany in 1917, primarily as a response to unrestricted submarine warfare by the Germans. Ohio, then the fourth largest state, made the fourth largest contribution in men to the Armed Forces in World War I, estimated at a total of 225,000 men.

The largest contingent of Ohioans was the famed 37th "Buckeye" Division, which fought at Mountfaucon, in the Meuse-Argonne Offensive, and in the Saint Mihiel Offensive. It was rated by the German General Staff as one of the five best American divisions. The Ohio National Guard also provided the 166th Infantry Regiment which served as part of the 42nd "Rainbow" Division, and one battalion of black soldiers which served in the 372nd Infantry Regiment.

During the period between the two World Wars, the Ohio National Guard was frequently called upon to perform relief duties during natural disasters. The greatest disaster in Northern Ohio since World War I was the great tornado of 1924 which struck the Lorain and Sandusky area with terrific force. Units of the National Guard saw considerable service in this area. Other major disasters have been the almost yearly floods of the Ohio River, those of 1936 and 1937 being especially severe.

WORLD WAR II
The year 1939 brought another World War. By the Summer of 1940, the federal government was calling troops into service for training to defend the nation. The Ohio National Guard was mustered into service at Camp Shelby, Mississippi in October 1940. The 37th Division had 9,000 Guardmembers, but by early 1941 had grown to nearly 20,000 soldiers.

The division served in the Pacific Theater, fighting on New Georgia, Bougainville, and the Philippines, where the division captured the heavily fortified city of Manila. Of 17 Congressional Medals of Honor awarded to Guard personnel, seven were awarded to members of the "Buckeye" Division. After the war, a separate Air National Guard was established in Ohio.

KOREA
The outbreak of the Korean Conflict provided another opportunity for Ohio's Guard to serve the nation. In 1952, the 37th Division was ordered to active duty as a training division at Camp Polk, Louisiana. More than 25,000 soldiers were trained by Ohio National Guard personnel, and many of the original members of the division were sent to Korea as individual replacements.

After the Korean Conflict, the National Guard was reorganized and given responsibility for air defense within the state. As a result, Ohio's anti-aircraft artillery battalions converted to missile units and manned the full-time missile sites in the greater Cleveland area.

In response to heightened tensions in Europe, President John F. Kennedy called up reservists to prevent a war. Ten Ohio units, six Air Guard and four Army Guard, were placed on active duty during this "Berlin Buildup" which was considered the most successful mobilization in the history of the National Guard, proving once again the value of the well-trained citizen soldiers.

VIETNAM
Under the Kennedy administration's flexible response theory, the National Guard was reorganized, a process that eventually led to the loss of 37 organizations from the Ohio National Guard. This included the famed 37th Division, which had served in two World Wars and the Korean Conflict. The 73rd Brigade was formed from the remains of the 37th Division and placed under command of the 38th Division headquartered at Indianapolis, Indiana.