There is a Torah obligation quoted in the Talmud [Sanhedrin 102.1] “One who approaches to kill you– pre-empt– and eliminate him!” One can understand two points can from this law:
 The legal decision refers to a situation when one indeed knows that he is approaching with the intent to kill you.
 Upon realizing that his intent is to kill you, one must not wait until he approaches you in combat, rather, pre-empt and eliminate him. These two points give us clear direction in our times. There are those who are in pursuit of peace that are convinced that since the time of Hitler and his professors– may their names be blotted out– no longer are there those who wish to annihilate us, there can no longer be a situation whereby a gentile would plot harm to the Jewish people. In any event, so they say, this is a situation of mere uncertainty.
How can one be certain what is in the heart of this gentile? Surely no-one can know what is stirring in the heart of another?! Regarding this situation the Torah says – by recognizing the overall circumstances one can indeed know with certainty that his intent is to kill you.
When one knows the education he has been subjected to; by knowing his activities of yesterday and of today– one can be certain without any shadow of doubt that his intent is “to kill you”.Now that one knows that he is approaching “to kill you”; do not wait until he approaches you in combat, for then, G-d forbid, it may be too late, rather it is imperative that you must pre-empt the situation and eliminate him. Those that act it this way – they must not be labeled as spillers of blood; but this is the legal imperative of Torah – “One who approaches to kill you – pre-empt – eliminate him!” In such situations one should go forth with absolute fortitude, fearless of nobody; not from “the rider” [terrorist] neither from “the horse” [his weapons], because “we go forth in the name of our G-d”, “your camp must be Holy”, then “will our G-d be present in our camp”; and when the enemy perceives this “the dread of the Jews will fall upon them”.
Public talk Shabbos Parshas Bereishis 1969