Very good, devilsadvocate. I agree that there can be occasional exceptions to virtually any particular rule anyone can lay down.
I have always felt that the Vietnamese invasion of Cambodia was, under the circumstances, the best thing that could have happened there at the time. The alternatives were absolutely horrible.
Vietnam's destruction of the Pol Pot regime, in fact, was an action which in the broad sense ended up being very much in favour of life and liveliness in that part of the World. Pol Pot's regime was about as opposed to life as any regime I've ever heard of.
I totally support Vietnam's pre-emptive attack on the Khymer Rouge, and it's about the only example of its sort I can think of that I do support.
I am less inclined to look favourably on the other 2 scenarios you cited, although I understand the argument in both cases.
A French-Polish attack on Germany was not justified.
But...if the British and French had supported Czechoslovakia more strongly in '38...or even if the Czechs had decided alone to stand and fight...the German Army generals would have arrested Hitler and Goering and shut down the Nazis, and that would have been the end of it. No war necessary at all in that case. It's recorded. It's a fact. And it's a great shame it didn't happen.
The Czechs had 88mm guns and excellent border defenses that would very likely have turned a German assault into a disaster, and the German military staff knew it. They were absolutely ready to overthrow the government at a moments notice if the order to attack was given. Hitler got lucky, very lucky, because the French, the British, and the Czechs lost their nerve. Too bad for Germany and Europe!
I do not support Israel's attack on the reactor.
So I guess Vietnam's strike on Cambodia remains the one and only pre-emptive strike in modern times that I do support wholeheartedly.