What happens if you fail the final mission when playing Ironman mode?

I'm playing through on Classic Ironman and it's been seriously tough so far. I've got my third best squad (please don't ask about the other two, it's still a sensitive matter) to the start of the temple assault, but haven't yet started it as I'm not sure what will happen if I fail. Is it totally game over? Or do I get the chance to train up a new squad a try again?