The last six days of Pesach are rather different to all our other festivals. The celebration is muted. There is a complete mood change from the wine and song of the Seder nights.

On these last days of Pesach we commemorate but we do not celebrate the Exodus from Egypt. Having left Egypt, the Children of Israel arrive at the Red Sea. They seem doomed. Pharaoh’s chariots are closing in on their rearguard and there can be no possible escape through the fierce currents of the Red Sea to the fore.

At the moment of greatest despair the Bible’s most iconic miracle offers them salvation. The waters of the Red Sea divide and a pathway appears for them to make their getaway. Once they are safe the waters come crashing down on the Egyptian army. You might reasonably think that it is time to pour out more wine and join together in song.

You would however be utterly wrong. We do not find joy from inflicting pain on our enemies. There is nothing to celebrate in the death of our foes no matter how much evil they visited upon us. The idea is expressed most explicitly in the Talmud: "The Egyptians were drowning in the sea. At the same time, the angels wanted to sing before God, and the Lord, God, said to them: 'My creations are drowning and you are singing!'" (Megillah 10).

The angels were silenced. Our festival celebrations are muffled. War requires inhumanity but Jewish writers across the ages and across the spectrum of opinion have been clear – we must never allow that inhumanity to demean us.

Philo of Alexandria wrote in first century BCE: "For though the slaughter of enemies is lawful, yet one who kills a man, even if he does so justly and in self-defence and under compulsion, has something to answer for, in view of the primal common kinship of mankind.”

We recognise these exact sentiments during the Seder as well. As we enumerate the Ten Plagues which were visited on the Egyptians, we spill a small amount of wine. We find no pleasure in the death of other human beings, whether friend or foe.

This anguish and hate of war has also expressed itself in Jewish literature as an extreme concern for the welfare of non-combatants who are sucked into the violence and suffering of battle. The medieval Jewish leader Nachmanides even insisted that the protection of enemy civilians is one of the 613 original mitzvot and “God commanded us …to deal with compassion even with our enemies even at time of war”.

It is of course tempting to try and apply this approach to the events of the last six months as some sort of criticism of the IDF or the policies of the Israeli government. To attempt this at this stage in our struggle for survival is, however, not only naïve and dangerous but both halachically and theologically mistaken.

During a war we must adopt a cruel pragmatism even at the expense of our most cherished values 

The halachah also recognises that while war is to be avoided, once the decision is made it must be prosecuted with one overarching aim in mind – to win. Many other values fall away. One of the most exalted mitzvot is that of ransoming captives who have been kidnapped. As Maimonides tells us: The redeeming of captives takes precedence over supporting the poor or clothing them. There is no greater mitzvah than redeeming” (Mishneh Torah, Hilchot Matanot Aniyim 8:10-11).

But in times of war this mitzvah is over-ruled by the superior principle that there is nothing more important than winning the fight. This was illustrated by a rabbinic dispute which erupted during the hijacking of several planes to an obscure airfield in Jordan called Dawson’s Field by the Popular Front for the Liberation of Palestine on the September 6 1970.

One of the victims was a pre-eminent American Torah scholar and yeshivah head, Rabbi Isaac Hutner. His students sought to collect funds to negotiate his release. A decision to prohibit such a project came from an unlikely source. Rabbi Hutner had been a student at the legendary Slobodka Yeshivah in Kovno, Lithuania in the early tears of the 20th century. There he would have met another student, Rabbi Yaacov Kaminetsky.

Both men emigrated to New York where they became leading figures in the world of the major yeshivot of the United States. But it was Rabbi Kamenetsky who forbade the attempt to ransom his old friend. The argument as explained by the current head of Yeshiva University, Rav Herschel Schachter, was: “One cannot redeem captives in this situation because then we would be helping our enemies in the middle of the war, since this large monetary gift to our enemies would allow them to further strengthen their position in the war.”

The principle we derive from this is that during a war we must adopt a cruel pragmatism even at the expense of our most cherished values. That is not to say that we abandon those values. As best we can we remain faithful to those ideas which have distinguished us among the nations of the world but wars must not be lost.

Israel has a proud record in this regard. After all the battles have been won, our independent press and judiciary reflect on the conduct of the fighting to hold our soldiers to account in a manner unlike any other nation. Yet during the heat of battle our greatest obligation is to win. There is no greater ethical imperative.