As the Second World War progressed, Darby saw rapid promotion to the grade of lieutenant colonel. He was with the first U.S. troops sent to Northern Ireland at the outbreak of the war, and during his stay there, he became interested in the British Commandos. His interest was such that, when the U.S. Army decided to establish its Ranger units, he was assigned to direct their organization and training. Many of the original Rangers were volunteers from the Red Bull, the 34th Infantry Division.

“Darby’s Rangers” trained with their British counterparts in Scotland and in 1943, the 1st Ranger Battalion made its first assault at Arzew. Darby was awarded the Distinguished Service Cross (DSC) for his actions during that operation. The citation stated:

“Lt. Col. Darby struck with his force with complete surprise at dawn in the rear of a strongly fortified enemy position. Always conspicuously at the head of his troops, he personally led assaults against the enemy line in the face of heavy machine gun and artillery fire, establishing the fury of the Ranger attack by his skillful employment of hand grenades in close quarter fighting. On March 22, Lt. Col. Darby directed his battalion in advance on Bon Hamean, capturing prisoners and destroying a battery of self propelled artillery.”

The 1st Ranger Battalion saw further action in the Italian Campaign. Darby received a second award (oak leaf cluster) of the DSC in 1943 for his gallantry in Sicily:

“Lt. Col. Darby, with the use of one 37mm gun, which he personally manned, managed not only to repulse an enemy attack, but succeeded with this weapon in destroying one tank, while two others were accounted for by well directed hand grenade fire.”