Excellent Advice on Non-Violence from Simone Weil

Simone Weil, Gravity and Grace, tr. Emma Craufurd, Routledge 1995, p. 77, emphasis added.

Non-violence is no good unless it is effective. Hence the young man's question to Ghandi about his sister. The answer should have been: use force unless you are such that you can defend her with as much chance of success without violence. Unless you possess a radiance of which the energy (that is to say, the possible effectiveness in the most material sense of the word) is equal to that contained in your muscles.

We should strive to become such that we are able to be nonviolent.

This also depends on the adversary.