When lives are at stake, Judaism permits--and probably requires--fighting.
He also notes, "There is an opinion that in our times, even when the foreigners come only for monetary gain, we sally forth against them on the Sabbath, for if the Jews do not let the non‑Jews despoil and plunder their possessions, they will kill the Jews." (Shulkhan Arukh, Orah Hayyim 329:6, 7)
In sum, even though the Talmud and codes do not directly connect communal self‑defense to the duty of either defending oneself or intervening on behalf of another, and even though one might argue that the latter duties do not establish the former, the Talmud does not question that a community must defend itself and even desecrate the Sabbath in the process. The codes not only endorse that right, but expand upon it.
These sources also do not insist on the usual procedures required for engaging in other types of war when it is a matter of communal self‑defense.
What About Preemptive Attacks?
To this core case of defensive wars we could add the category of obligatory wars, mentioned in the tractate Sotah [44b], undertaken for the purpose of "diminishing the heathens so that they do not march against them." This form of defensive war is preemptive in the sense that it is aimed not at an actual attack, but against those who might well attack.
Surprisingly, Maimonides omits this category in his summary of the law of war. This omission has led later commentators to speculate about the permissibility of preemptive defensive war. A range of opinions supports discretionary war to "diminish the heathens" provided the talmudic procedural requirements are satisfied.
Because these requirements, including consultation of the priestly breastplate, the urim vetumim, are inapplicable under modern conditions, the only relevant addition would be an obligatory war to defend Jews against both actual and potential aggression "...even when there is only a suspicion that they may attack us." (Arukh HaShulkhan He'Atid, Hilkhot Melakhim74:3-4)
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