Magnitude of blow dealt by defeat here indeterminate? (6,7)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

I knew nothing about this American battle, so read it up on Wiki. I am still none the wiser about how to parse this, except for the obvious “little / big” dichotomy leading to something indeterminate. Perhaps one of our Transatlantic solvers can explain it to us.