In 1917, during the Romanian Campaign of World War I, Grigorescu was the commander of the Romanian armies in the Battle of Mărășești. It was there that he achieved his greatest success, managing to defeat Imperial German troops led by General August von Mackensen. After the battle, Grigorescu was nicknamed the "hero of Mărășești". His victory was praised in Entente countries, and earned the general a sword sent to him by Yoshihito, the Emperor of Japan. He was also a recipient of the Order of Michael the Brave (the highest military decoration of Romania), 3rd Class in October 1916, and 2nd Class in February 1918, as well as the French Legion of Honour, Grand Officer Class.