After school, Dave Rosell enlisted to aid in the Civil War effort. There is not a lot known about his specific wartime experiences, but he was a member of the 13th Pennsylvania Colored Regiment in the 5th Brigade and acted as the Surgeon-in-chief. At the time of his retirement from the Union Army he had achieved the rank of Major. This likely was a defining time for Rosell's character as supporting those who fought against the slave-holding states was reasonably one of his most serious endeavors.
Marlene Parks, New York Central College 1849-1860: McGrawville, New York, Vol 11, Middletown De, 2017
Following his time in the Civil War, Rosell became a native of Philadelphia where he was seeking to become a physician. At this same time, he enlisted his medical capabilities in that of a National Guard Reserve regiment where he was the head surgeon. These regiments are of historical significance as it begun the introduction of a consistent African American presence in American Military units.
The Times (Philadelphia, Pennsylvania) · Mon, Nov 22, 1886 · Page 1