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It is a Jewish principle to give people the benefit of the doubt till they prove that they don't deserve such a benefit. Our enemies are not to be trusted and we must stand strong against them. But we also have our morals which make us better than they are. We cannot forsake our moral imperative because we are angry or tired.
I have only stated my opinions on this topic.
When yaakov's sons killed the men of shchem, that wasn't about morality/ western morality.
Rabbi Meir Kahane said something along the lines of "i'm tired of being more moral than they are"
(some say yaakov rebuked them for excess, others not. Rabbi Binyamin Kahane said not)
Rabbi Meir Kahane also pointed out in his debate with Dershowitz, that the Torah is not about morality. Killing Amelek, all of them, men women and children, is not moral.
there is the text in the shulchan aruch and talmud - that if the enemy come , even just to take straw, you go to war with them.
Here is something easily found with google regarding that
google enemy straw war torah
When it is a city close to the border, then, even if they want to come only for the purpose of [taking] straw and stubble, we desecrate the Shabbos because of them; for [if we do not prevent their coming] they may conquer the city, and from there the [rest of the] land will be easy for them to conquer.
(Shulchan Aruch, Orach Chaim 329:6)
(found from this site
http://www.truepeace.org/ihalachah.htm though I don't know about rest of the site.. I picked up the passage via google )
Here is a site I have used a few times, it speaks of the same thing, but references the talmud.
http://www.myjewishlearning.com/ideas_belief/warpeace/War_TO_Combat/War_Types_Hartman/War_Defensive_Dorff.htmgives a description, and mentions talmud tractate eruvin 45b
Rabbi Meir Kahane has also said, in audio radio show, when asked if he believes in an eye for an eye. He had to give succinct answers. He very cleverly responded "I believe in two eyes for an eye".