Romania was neutral for the first two years of World War I, entering on the side of the Allied powers on 27 August 1916. The start of the Romanian Campaign of 1916 found Moșoiu as commanding officer of 2nd Regiment Vâlcea and commander of the military group assigned to the Olt–Lotru sector, when he played a decisive role in the Sibiu Offensive. From 26–29 September the 3rd Infantry Brigade under his command fought tenaciously in the Olt Valley with the 1st Army Corps led by General David Praporgescu, blocking an attempted breakthrough of the 9th German Army under the command of Erich von Falkenhayn. In October 1916 he took command of the 23rd Infantry Division. In January 1917 he was promoted to brigadier general and put in command of the 12th Infantry Division. For his bravery during the Battle of Transylvania in the fall of 1916 he was awarded on 2 January 1918 the Order of Michael the Brave, 3rd Class.