During the Romanian Campaign of World War I, Cristescu was Chief of Staff of the 2nd and 3rd Romanian Armies and later of Army Group General Averescu. From 10 November to 5 December 1916 he was Commander of the Northern Army. In 1917 he was promoted to major general. In June of that year he took command of the Romanian 1st Army, during the operations leading up to the Battle of Mărășești, against the German 9th Army. The German offensive began on 24 July with a sustained artillery bombardment. Despite the failure of large Russian units that refused to fight and retreated, the 1st Army together with the remaining troops of the Russian 4th Army fought back. Cristescu took upon himself the difficult task of reconstituting the front line; a single front was created through fierce fighting, supported by a strong artillery barrage, and by 6 August the German attack was repulsed. He continued to lead the 1st Army Mărășești until 11 August, when he was succeeded by General Eremia Grigorescu.