... Fredericksburg?
That's what I thought initially as well (Burnside, Fredericksburg). But then I remembered that Hooker did order General Sedgewick to assault Marye's Heights to try to pin down Confederate forces there (Early?). Sedgewick initially did reasonably well until Hooker's forces were defeated and Lee was able to send reinforcements to Marye's Heights.
So I figured that that's maybe what IronJack was talking about ...