The retreat and destruction of Elphinstone's army during the First Anglo Afghan war.
Firstly, a show of British force following the murder of Sekundar Burnes may have quelled the Afghans, but the practically infirm Elphinstone instead wasted the time by dithering in the British cantons.
He then lost a party of political officers in a meeting with tribe leaders and did nothing.
The next bit can't quite be all Elphinstone's fault. The British army was meant to be granted safe passage through the Khyber by Akbar Khan, instead, throughout the passage through the frozen Khyber, the army was relentlessly hounded by the ghazis, losing huge numbers to both the tribesmen and the snow. Elphinstone, of course, did nothing.
Then Akbar Khan came forward again and took the married officers and their wives under his protection. While I don't believe anything ill happened to these prisoners, their loss did weaken the army further.
Perhaps the full destruction of the army could be seen with the last stand of the 44th East Essex. In the end, as I recall, only one man made it to the Army's destination (which I forget but I think it was Kandahar.)