A bridge too far

Taking on more than you can likely achieve, particularly too many tasks. While it can imply a long-shot, it connotes more an idea of striving to do more than can be achieved, rather than striving to do a thing that is unlikely to be successful. The previous definition by "A solid cube of tungsten" had a good historical reference, but the particular example was not quite on point: "The story behind it is that during WWII in 1944, the Allies came up with a plan (Operation Market Garden) to use paratroopers to secure three bridges over three key rivers behind enemy lines in The Netherlands. British forces would then rush forward across all three bridges to relieve the paratroopers, and later push into Germany itself. It was an extremely ambitious plan. A British commander is alleged to have said “I think we may be going a bridge too far”, and he was right. The American airborne divisions were able to capture the first two bridges, and were relieved. But the British and Polish paratroopers at the third and final bridge at Arnhem were basically wiped out before tanks of the Irish Guards could punch through to reach them. Market Garden was an operational failure."