By March 1945, the Allied armies had advanced into Germany and had reached the River Rhine. The Rhine was a formidable natural obstacle to the Allied advance, but if breached would allow the Allies to access the North German Plain and ultimately advance on Berlin and other major cities in Northern Germany. Following the "Broad Front Approach" laid out by General Dwight D. Eisenhower, the Supreme Allied Commander of the Allied Expeditionary Force, it was decided to attempt to breach the Rhine in several areas. Montgomery, commanding the British 21st Army Group, devised a plan to allow the forces under his command to breach the Rhine, which he entitled Operation Plunder, and which was subsequently authorised by Eisenhower. Plunder envisioned the Second Army and the US Ninth Army (Lieutenant General William Simpson) crossing the Rhine on three fronts; at Rees, Wesel and in the area south of the Lippe Canal. To ensure success, Montgomery insisted that the amphibious assaults be supported by an airborne landing, which was code-named Operation Varsity. Varsity was initially planned with three airborne divisions in mind, with all three to be dropped behind German lines in support of 21st Army Group as it conducted its amphibious assaults to breach the Rhine. However, during the earliest stages of planning, it became apparent to the planners that the 13th Airborne Division would be unable to participate in the operation, as there were only enough combat transport aircraft in the area to effectively transport two divisions. The plan for the operation was therefore altered to accommodate the two remaining airborne divisions, the British 6th Airborne and the US 17th Airborne Division. The two airborne divisions would be dropped behind German lines, with their objective to land around Wesel and disrupt enemy defences to aid the advance of the Second Army towards Wesel.