Between 1937 and the beginning of World War II, Dumitrescu served as a military attaché in both Paris and Brussels.

In September of that year, Dumitrescu repulsed a Soviet attempt to cross the Dniester in the east, behind the Eleventh Army.

After Adolf Hitler convinced Ion Antonescu to continue the war beyond Romania's pre-1940 borders, Dumitrescu then led the Third Army to the Crimea, taking part in the Battle of the Sea of Azov.

By October 10, the Third Army had marched 1,700 kilometres from Romania, fought four major battles and 42 minor engagements.

This was ameliorated, to a certain degree, by the High Command's decision to incorporate all Romanian forces in the southwest of the Soviet Union into the Third Army.

In November 1942 the Red Army launched a devastating attack in the southwest, breaking through the Romanian line and forcing Dumitrescu into retreat.

By this time, however, as with the rest of Romania, Dumitrescu turned against Nazi Germany and captured more than 6,000 German prisoners of war.