The most significant military action on the Rhine River during World War II was Operation Plunder, a massive Allied offensive launched in March 1945. Directed by Field Marshal Bernard Montgomery, it involved the British 21st Army Group and the U.S. Ninth Army crossing the river to establish a firm foothold in the German heartland. This operation was one of the largest amphibious assaults in history, second only to D-Day, and was preceded by a staggering four-hour artillery barrage from over 4,000 guns. A key component of this offensive was Operation Varsity, the largest single-day airborne operation ever conducted, which dropped thousands of paratroopers behind enemy lines to disrupt German defenses. Simultaneously, General George S. Patton's Third Army made a surprise crossing further south at Oppenheim. These combined efforts broke the final major natural barrier protecting Nazi Germany, allowing Allied forces to pour into the Ruhr industrial region and effectively sealing the fate of the Third Reich. The engineering feat was equally impressive, with Allied engineers building massive tactical bridges, such as the 1,152-foot M2 treadway bridge near Wesel, in record-breaking time under heavy fire.