On the afternoon of Sunday 17 September British tanks advanced into Holland in concert with 1,534 transport aircraft and 491 gliders. Their objective was a series of bridges across the Rhine, possession of which would allow the Allies to advance into Germany. In the event the operation was dogged by bad weather, flawed planning, tardiness and overconfidence, and ended with the Arnhem crossing still in German hands despite an epic nine-day battle that cost the British 1st Airborne Division over two-thirds of its men killed, wounded or captured. Here is what happened, hour by desperate hour.