The Battle of Remagen was an 18-day battle during the Allied invasion of Germany in World War II from 7 to 25 March 1945 when American forces unexpectedly captured the Ludendorff Bridge over the Rhine intact. They were able to hold it against German opposition and build additional temporary crossings. The presence of a bridgehead across the Rhine advanced by three weeks the Western Allies' planned crossing of the Rhine into the German interior. After capturing the Siegfried Line, the 9th Armored Division of the U.S. First Army had advanced unexpectedly quickly towards the Rhine. They were very surprised to see one of the last bridges across the Rhine still standing. The Germans had wired the bridge with about of demolition charges. When they tried to blow it up, only a portion of the explosives detonated. U.S. forces captured the bridge and rapidly expanded their first bridgehead across the Rhine, two weeks before Field Marshal Bernard Montgomery's meticulously planned Operation Plunder. The GIs' actions prevented the Germans from regrouping east of the Rhine and consolidating their positions. The battle for control of the Ludendorff Bridge caused both the American and German forces to employ new weapons and tactics in combat for the first time. Over the next 10 days, after its capture on 7 March 1945 and until its failure on 17 March, the Germans used virtually every weapon at their disposal to try to destroy the bridge. This included infantry and armor, howitzers, mortars, floating mines, mined boats, a railroad gun, and the giant 600 mm Karl-Gerät super-heavy mortar. They also attacked the bridge using the newly developed Arado Ar 234B-2 turbojet bombers. To protect the bridge against aircraft, the Americans positioned the largest concentration of anti-aircraft weapons during World War II leading to "the greatest antiaircraft artillery battles in American history". The Americans counted 367 different German Luftwaffe aircraft attacking the bridge over the next 10 days.