Forbes entered the Union Army with his brother and served with distinction until the end of hostilities in 1865. At the age of 18, he was captured by Confederate forces and was held in a prison camp for four months. After his release he was hospitalized for three months while recovering from scurvy and malaria. He then rejoined his regiment and was promoted to captain at age 20.

In 1867, Forbes invested in a strawberry farm in Carbondale, Illinois, which,