For neophytes to the study of the American Civil War, the function played by the pre-war Federal Regular Army before, during and after continues to reside a bit in obscurity.  Officers from the antebellum army played large roles on both sides during the war, though they rarely achieved greatness remaining a part of the Federal Regular Army, even if staying true to their oath as army officers.  Those officers usually took up new commissions in the many militia units raised by the different States – again on both sides.  Rapid promotion and the chance to play much larger roles than they played before the war represented key factors in the exodus.  Which comes back to the question, what role did the Regular Army play during the war?