On August 13, 1861 George joined the 48th New York Regiment as a private and fought at the capture of Fort Walker and Fort Beauregard.
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When the Civil War began in early 1861, he was commissioned a first lieutenant in the Union Army (Joel Snyder, O'Sullivan's biographer could find no proof of this claim in Army records) and, over the next year, was present at Beaufort, Port Royal, Fort Walker, and Fort Pulaski.