I don’t have the exact quote handy, but W. Churchill thought about this situation a good deal, and said to the effect that it is best to stop a foe, when he is of a small threat and your chances are good. Next best is to stop your foe when he grows to be a big threat, and your chances are slim. Avoid a situation where your foe has grown to the point of your chances being nil. But, in any case, it is better to fight your foe with just cause on your side, against all odds, rather than be enslaved (or dead).
If you will not fight for the right when you can easily win without bloodshed, if you will not fight when your victory will be sure and not too costly, you may come to the moment when you will have to fight with all the odds against you and only a small chance of survival. There may even be a worse case: you may have to fight when there is no hope of victory, because it is better to perish than to live as slaves.
— Winston Churchill