That was a big issue for the North in the early years of the Civil War. Southerners were disproportionately represented in the small pre-war standing army and West Point alumnae. So the war starts, and they mostly go home and switch sides; at the same time the northern armies are massively expanding, with a gutted officer pool.
Linguistic quibble: "Alumnae" is the nominative feminine plural of the noun "alumnus", and thus refers specifically to a female graduate of some institution of higher learning, &c. In 1860, there was no such thing as an alumna of West Point, a school which at that time had only alumni (which is the nominative masculine plural, and the correct usage in this case).
