I would suggest worrying less about the legality of preventive war, and more about the inevitability of it in the face of certain forms of failed deterrence.
When two parties are at peace, but in high distrust of each other (Think Iran-Israel or Pakistan-India for current examples) deterrence becomes a very careful balancing act. Being balanced are an array of provocative capabilities and postures to maintain a zone where each side's assessment of the cost-benefit of war vs peace leads them to believe that the best result comes from peace. When something disrupts that balance in a way that significantly shifts the cost-benefit calculus, the side that feels that a potentially overwhelming attack is inevitable is "provoked." This is likely to result in an act of preventive war in an attempt to re-balance the scales of deterrence.
So, if Iran were to develop nuclear weapons, how does this affect Israel's C/B analysis of deterrence? It may well provoke them in that it causes them to believe that their best chance for peace, or perhaps even survival, is to conduct preventive war.
Similarly between Afghanistan and Pakistan. Both sides being already nuclear, it is then other factors that could suddenly change and affect the balance of their deterrence. This is the all too downplayed reality of US meddling in Afghanistan and Pakistan in pursuit of Al Qaeda. What affect do our actions have on Pakistan-India deterrence? How much do we even consider such issues when we are totally consumed by our own interests and our own fears being serviced there currently? If something happened, I suspect the U.S. would be like the bumbling, but well-intended Steve Urkel of the old "Family Matters" TV series and ask "Did I do that???"
We dwell on the Iranian nuclear issue a great deal (too much IMO), when it fact, it may actually lend greater balance to that particular deterrence equation. Yet we completely ignore the delicate balance between Pakistan and India that we have already tipped in India's favor by our very actions in Afghanistan and the FATA. Pile on top of that the President's recent trip and broad assurances to India and that scale tips even more. Not saying anyone needs to be alarmed, but it is certainly something we should be extremely aware of and take fully into account when weighing various COAs.
Legal vs illegal is a nicety of civilized society. It is quickly trumped by perceptions of national survival.
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