"What should we have done once confronted with a Hitler" is the wrong question. It's really not a question. Whether or not it is set intentionally, it's really a trap that is generally understood to be a proof of the necessity of "just war."
The necessary question is how do we treat our enemies? Violence begats violence, oppression leads to revolt, hoarding inspires theft, regime change and monopolizing resources creates poverty and powerlessness which inspires terrorism.
The question isn't whether or not force and coersion are necessary; that's not really a question either, because we all assume that it is necessary, whether we want to protect our advantage or over throw the advantaged.
The actual question is whether preemptive peace is possible. Can we learn from our habitual mistakes and set a different course that probably won't deliver immediate results, but with vision and courage could create a world we can't yet imagine that our grandchildren could shape.
