In the weeks following D-Day, German troops began retreating en masse, as Allied forces advanced across France, Belgium, Luxembourg and the Netherlands. By September 1944, however, the overstretched Allies were approaching formidable German defenses along the Siegfried Line, which had held strong since World War II began.

British Field Marshal Bernard Law Montgomery came up with a daring plan to bypass the Siegfried Line by crossing the lower part of the Rhine River, liberating and driving into the industrial heartland of northern Germany.

Code-named Market Garden, the offensive called for three Allied airborne divisions (the “Market” part of the operation) to drop by parachute and glider into the Netherlands, seizing key territory and bridges so that ground forces (the “Garden”) could cross the Rhine.

But controversial decisions and unfavorable circumstances began stacking up from the start of Operation Market Garden. Despite their heroic efforts, the Allied forces ultimately failed to achieve their objectives—and sustained devastating losses in the process. Here's what went wrong:

On the morning of September 17, 1944, three divisions of the First Allied Airborne Army—the U.S. 101st and 82nd Airborne and the British 1st Airborne—began flying from bases in England across the North Sea to the Netherlands. The 101st Airborne was tasked with capturing Eindhoven, as well as several bridges over the canals and rivers north of that town, while the 82nd Airborne was ordered to capture territory around Nijmegen, including a key bridge over the River Waal.

Some 10,000 British and Polish troops of the British 1st Airborne (nicknamed the “Red Devils”) had the most difficult task: capturing and holding the northernmost bridge over the lower Rhine at Arnhem. German anti-aircraft defenses around Arnhem itself were thought to be too strong, and the troops were dropped up to eight miles away, despite warnings from some Allied planners that a small “coup de main” party should land on the bridge itself.

Only a single battalion of the 1st Airborne (fewer than 800 men) managed to reach the Arnhem bridge, while the Germans forced the rest into a pocket near the village of Oosterbeek, several miles away.

Due to limited numbers of transport aircraft, the British forces at Arnhem had to be dropped into the Netherlands over three days, rather than all at once, lessening the possibility of surprise as well as the impact of the attack.

While many troops from the 1st British Airborne were dropped by parachute and gliders on the afternoon of the first day (September 17), the 4th Parachute Brigade and the rest of the glider troops didn’t arrive until the following day, and the Polish brigade was still more delayed.

Dense fog in England on the second day of the operation, as well as thick, low clouds over the battleground in the Netherlands, hampered the transport of troops, as well as supplies. The supplies would have been crucial to the survival of British forces fighting to hold Arnhem Bridge.

To make matters even worse, the wooded landscape and the separation between the different British battalions meant many of their radios stopped working. These failures broke down communication and made it difficult for the 1st Airborne Division and its commander, Major-General Robert “Roy” Urquhart, to coordinate the attack on Arnhem.

According to historian Antony Beevor, Urquhart’s signals officers had anticipated problems with their radios before the operation, and Urquhart himself had expressed serious doubts about Operation Market Garden, reportedly calling it a “suicide operation” just two days before Allied planes left for the Netherlands.

By the end of the first day of Operation Market Garden, the 2nd Battalion of the 1st British Airborne, commanded by Lt. Col. John Frost, had reached the north end of Arnhem bridge and fortified themselves within nearby homes, preparing to hold the bridge on their own until the arrival of relief ground troops.

But the ground relief column, led by XXX Corps, had run into its own problems: The road toward Arnhem was narrow, only wide enough for two vehicles, and German infantry men wielding Panzerfaust anti-tank weapons picked off the nine lead British tanks right at the start of their advance. Allied ground troops managed to advance only seven miles by the end of the first day.

On the second day (September 18) they covered 20 miles and caught up with U.S. troops near Eindhoven, which the 101st Airborne had managed to liberate from German control. Though they fought their way across the Waal by September 20, they were still eight long miles away from helping their desperate British comrades at Arnhem.

Before Operation Market Garden even started, Allied intelligence got reports that two well-equipped German SS Panzer (tank) divisions were in the area around Arnhem. But commanders of the operation, including Lt. Gen. Frederick “Boy” Browning, decided the operation should go ahead anyway—a risk that turned into a disaster for Allied troops at Arnhem.

The slow advance of the XXX Corps gave Germany time to strengthen its defenses, confront the advancing ground troops at Nijmegen, and subject the lone British battalion at Arnhem to a crippling onslaught, which they resisted fiercely before submitting on the fifth day of the battle. With the main objective of the operation lost, more than 3,000 British troops dug in at Oosterbeek until September 25, when they were forced to begin evacuating across the Rhine.

Though Operation Market Garden liberated much of the Netherlands from Nazi occupation, established a foothold from which the Allies could make later offensives into Germany and showed the courage and determination of the Allied forces in Arnhem, it remained a costly failure, with lasting consequences.

Of the approximately 10,600 Allied forces who made it north of the Rhine in September 1944, some 7,900 were killed, wounded or taken prisoner. Allied casualties during the operation totaled more than 17,000, compared with around 8,000 on the German side.

If Operation Market Garden had succeeded, World War II might well have ended in Europe before Christmas of 1944, with the Western Allies marching triumphantly into Berlin. Instead, the conflict would drag on for five more months after that date. Not only that, but it would be Soviet troops who claimed Berlin in May 1945, a difference that would prove decisive for the future of post-war Europe.