The defense heritage of Cassidy & Mueller P.C. began in Europe in 1918 when a young law school graduate named John E. Cassidy was assigned to the Hartford Brigade during World War I. There he met and befriended members of the insurance and insurance claims industry. After the war Cassidy opened the first down state claims office for Aetna Casualty & Surety Company in 1921. Two years later he began a private practice with Aetna Casualty & Surety Co. and Hartford Insurance Group as principal clients.