Well the success of non-violence and non-cooperation also depend on the support it has.One of Gandhi's greatness was the way he could galvanize the whole of India into following whatever he did.It is a remarkable thing taking into account the diversity of the India and the limited means of communication available then.
And non-violence and non-cooperation are the tools used when you already have the enemy already occupying you.With regards to external agression the army is the way to go.
Regarding the nazis Gandhi was supportive of the british alliance in the second world war.Though the british were the enemy at home he recognized that the british losing to the nazis was a worser thing.Thus he did not convey any objection when the british sent several Indian soldiers to fight in thre war.