The United States and its allies are engaged in an aerial war, for now at least, against the so-called Islamic State in Iraq and Syria, the paramilitary terrorist group better known as ISIS.

If the world ran according to halachah, would attacking ISIS halfway across the world be permissible? On the surface, the answer would seem to be a resounding yes, but where human life is concerned, there is no such thing as an easy answer in Jewish law.

Obviously, a terrorist may be considered a “rodef,” a “pursuer” who seeks to kill another person. On this, the law is clear. Based on Exodus 22:1, a talmudic sage, Rabbi Shila, pronounced the general principle, “If he comes to kill you, arise and kill him.” (See the Babylonian Talmud tractate B’rachot 58a.)

If a third party can prevent the rodef from committing the crime, this too is permissible. In fact, the Talmud bends the law to make it easier for someone to decide to be a rescuer. It does so by absolving that person from responsibility for any inadvertent property damage resulting from the attempt. (See BT Bava Kamma 117b.) This “is not the law,” the Talmud declares, “but if you did not say this [that the rescuer was free from liability for property damage], you wouldn’t [find] a man willing to rescue his fellow from the pursuer.”

Clearly, this has application in the case of aerial attacks of all kinds (drone, missile, or even a so-called smart bomb), because the property of innocent parties may be damaged, especially given the proclivity of such terrorists to operate in the midst of innocent civilians. Thus, the “rescuer” is free from liability.

The Talmud is also willing to bend the law regarding the need to warn the pursuer that he is engaged in conduct that could lead to a capital offense. (See BT Sanhedrin 72b.)

On the other hand, killing the rodef is the last resort, not the first. If another, nonfatal method would have worked, killing the pursuer can bring harsh punishment on the intended victim, according to some authorities. Thus, in BT Sanhedrin 57a-b, we are told: “If he pursues his fellow to kill him, and [the intended victim] could have saved himself by maiming a limb [of his pursuer], and did not so save himself [but killed him instead], he is executed for [murdering his pursuer].”

This is not a universally held opinion, but it does show the extent to which human life is valued.

The principle of pikuach nefesh (threat to life) looms large in Judaism. Virtually nothing – not even Shabbat or the laws of kashrut – takes precedence over it. In only three categories does pikuach nefesh take a back seat: committing an act of apostasy, sexually violating another person, and committing murder. If someone insists that you either commit such an offense or be killed, you must allow yourself to be killed. (See BT Ketubot 19a.) This is not absolute in the case of idolatry, but it is absolute in the other two instances.

The real problem, however, is not whether it is permissible to kill a pursuer, but in knowing whether the person killed indeed was a pursuer. By definition, a pursuer must be seen pursuing his intended victim (but see the note that follows this paragraph). This is not the case when terrorist bases are attacked, and certainly not when the attack comes from the skies. The attacks are based on circumstantial evidence alone – and that simply will not do when it comes to committing an act of justifiable homicide. “In capital cases, we do not allow conjecture,” the Talmud says. (See BT Sanhedrin 37b.)

It is not enough to believe a particular base is filled with terrorists who intend to take human life. The evidence has to be irrefutable, and that is not possible in such cases.

Even more to the point, while it is permissible to pursue a pursuer and to kill him, if necessary, wholesale attacks on terrorist bases inevitably lead to the shedding of innocent blood along with the guilty – the euphemistically titled “collateral damage.”

It would be comforting to be able to say without hesitation that it is not permissible to kill innocent people in pursuit of a pursuer.

One cannot say that, however, because (as always) halachah is not black and white. While it is protective of human life, Jewish law accepts that there are times when it cannot interfere. Instead, it bows to “the king’s law.” Thus, Maimonides explains in his Mishneh Torah, The Law of Kings, 3:10:

“A murderer against whom the evidence is not totally conclusive…, the king has permission to kill him and to repair society according to the needs of the moment. He may kill many on a single day, hang them, and leave them hanging for many days, so as to instill fear and stay the hand of the wicked of the world.”

There is no need for a trial; no need for witnesses; no need for conclusive proof. The purpose is not punishment, but deterrence, and if “the hand of the wicked of the world” is stayed, collateral damage could be seen as a fair price to pay.