Ioanițiu was born in Botoșani, in northern Moldavia. He attended the military school in Iași, graduating as second lieutenant in 1910 and lieutenant in 1913. After Romania entered World War I on the side of the Allies, he fought in the Romanian Campaign of 1916 in Dobruja and at the Battle of the Argeș. He was promoted to captain in 1916 and major in 1917, and fought at the Battle of Mărășești.