Pyrrhic victory \PIR-ik\, noun:

A victory achieved at great or excessive cost; a ruinous victory.

Technically it was a victory for the British, who attacked the patriot fortifications -- but a Pyrrhic victory if ever there was: out of 2,200 British soldiers 1,034 were killed or wounded, including one in nine of all the officers the British lost in the whole war.
-- Geoffrey Wheatcroft, "A Revolutionary Itinerary", The Atlantic, April 2001