In 1845 Monstery accepted a commission as a fencing instructor to the Russian Army, but was forced to retire due to injury and moved to Copenhagen where he killed a man in a sword duel. Fleeing Denmark to avoid being arrested, he moved to Baltimore and then unsuccessfully attempted to gain a commission in the US Army as a bayonet and sabre fencing instructor. Enlisting in the US Navy he joined the crew of the gunboat Vixen and thus took part in General Scott's landing at Veracruz during the Mexican–American War.