Carlson was ordered to Nicaragua in 1930 as an officer in the Guardia Nacional. A first lieutenant at the time, he earned his first Navy Cross for leading 12 Marines against 100 bandits in a night attack to break up a threat to his garrison. He was also commended for his actions following the 1931 earthquake at Managua, and for performance of duties as Chief of Police in 1932 and 1933.