From Ohio History Central
Allen County is named for Colonel John Allen, a participant in the War of 1812. Residents established the county on February 12, 1820. The first white settlement in the county was Fort Amanda, constructed during the War of 1812 to protect Ohio from British invasion and Indian attacks.
Allen County is located in the northwestern portion of Ohio. Much of the county used to be part of the Great Black Swamp. Allen County covers 404 square miles of land. The county seat is Lima, which also is the most populous community, with over forty thousand residents in 2000. The next closest city in population is Delphos, with just over 3,900 residents in 2000.