In March 1945, the rapidly advancing U.S. Army’s 9th Armored Division — to its surprise — found itself at Ludendorff Bridge over the Rhine at Remagan; one of the two surviving bridges into the heart of Germany. The bridge was a considerable prize and its capture would shorten the war. The Germans, desperate to stop the offensive, threw everything that had at the bridge in repeated attempts to destroy it.
German troops rigged the Ludendorff Bridge with explosives, which only partially detonated — failing to bring down the structure. Ground attacks, artillery and a 24-inch super-heavy Karl-Gerät mortar also failed. U.S. troops even captured a barge full of German soldiers carrying explosives as they floated down the river toward the bridge.
Finally, and incorrectly believing the bridge had been destroyed, German commando Otto Skorzeny ordered Waffen SS frogmen to float down the river using oil drums, with their objective to blow up a nearby pontoon bridge. Along the way, beams from extremely bright Canal Defense Lights attached to American M3 Grant tanks illuminated the group — exposing them to U.S. fire which killed two frogmen. U.S. troops captured three others.
It was the first time the Canal Defense Light saw combat. The term “Canal Defense Light” itself was an odd one for giant lamp attached to a battle tank, but that was deliberate — an anodyne-sounding codename to confuse spies. First developed by Greek inventor Marcel Mitzakis, the British War Office took interest, ordering 300 lights for trials beginning in 1940.
The actual devices themselves used a series of mirrors to reflect a powerful arc-light beam through narrow slit — making them challenging to disable with fire.
via the Tanks Encyclopedia. At top — an M3 Grant with a dummy turret gun and Canal Defense Light. U.S. Army photo
The British first used Matilda II tanks for the lights, and later, twin-cannon M3 Grant tanks supplied under Lend-Lease. Mitzakis derived the idea with Royal Navy Cmdr. Oscar De Thoren, inspired by naval searchlights. Their connection to the British military industry came through J.F.C. Fuller, an occultist-inspired military theorist and proponent of psychological warfare whose fascist sympathies later precluded him from the war effort.
The Canal Defense Light was extremely bright at some 13 million candlepower — far surpassing the U.S. Army’s “ambush light” deployed in Vietnam. The M3 Tanks with the blinding illuminators came to be known as CDL Tanks or “Gizmos,” with the CDL inside a rebuilt turret. One advantage of using the Grant tanks is that the tank’s main 75-millimeter cannon sat in the hull, allowing it to keep its main weapon.
The War Illustrated, a popular war-era magazine, reported that the lights did not live up to all of their promises. But they still had their uses — if they only saw limited service and only late in the war. During the crossing of the Rhine, CDL Tanks helped blind German defenses on the east bank of the river.
When C.D.L. tanks were used to light up a wide front they could turn night into local day and thus enable the pursuit of a defeated enemy to continue throughout the 24 hours. The aimed fire-power of the tanks themselves, even if not augmented by infantry, ordinary tanks and artillery, was impressive: direction-keeping on a night advance or assault was greatly facilitated and might prove decisive against an enemy on the run.
The U.S. military later ordered CDL Tanks for the invasion of Okinawa, but they arrived too late to participate in the battle and saw no further action.