Hmmm.

OK - so in order to qualify, I'd guess you'd need to have been a command officer...

Ambrose Burnside commanded in one massive debacle; Pyrrhus of Epirus has achieved everlasting fame for his ineptitude. On the other hand, Leonidas of Sparta lost his entire force with no gain of ground; mere numbers are insufficient.

Hitler and Stalin both attempted command control of their militaries during WWII; it's possible that each would qualify. Stalin, however, did win his war... and Hitler, despite his myriad failings, did exhibit genius...

So here's my nominees:
- F.M. Haig [UK; WWI]
- Guy de Lusignan, King-Consort of Jerusalem
- Gen. James Ledlie [USA; Battle of the Crater]
- Philip VI of France [Agincourt]
- Maj. Gen. William Elphinstone
- Lord Chelmsford [Zulu War]
- President Lyndon Johnson [USA; Vietnam]
- Gen. Arthur E. Aitken [UK; Battle of Tanga]
- Gen. Kuropatkin [Russo-Japanese War]
- King Ethelred ("the Unready") of England

I'm deliberately ommitting such notables as Custer (who only lost his LAST battle), McClellan (incompetent, but led no slaughters), and Montgomery (he was surprisingly effective).