Some years ago, Radio 4 ran a series of programmes presented by Michael Portillo called 'Things we forgot to remember' which looked at forgotten aspects of British and world history. The title of this series came back to my mind this week as I looked at how people are responding to Israel's current military activity in Gaza.

One thing that particularly struck me is how we seem to have almost totally forgotten that Allied military activity in the Second World War, a war that is generally regarded as a conflict necessarily fought for a legitimate cause, resulted in large numbers of civilian casualties.

Two examples will serve to illustrate this point.

First, Operation Chastise, the 'Dambuster' raid of 16 and 17 May 1943 in which the RAF attacked the Möhne, Edersee and Sorpe dams in western Germany in an attempt to disrupt German industrial production, and therefore the German war effort, is normally remembered as a celebrated feat of British arms. What is all too often forgotten is that the subsequent flooding resulted in an estimated 1,600 civilian casualties - 600 Germans and 1,000, mainly Soviet, enslaved labourers.

Secondly, fictional representations of Operation Overlord, the battle for Normandy in June-August 1944, such as the films The Longest Day and Saving Private Ryan, generally depict the fighting in Normandy as if the only casualties involved were those suffered by the soldiers of the Allied and German forces. This was not the case. It has been estimated that around 19,980 French civilians were killed as a result of the battle. The historic town of Caen was particularly badly hit with some 80 per cent of the city being destroyed and some 3,000 civilians being killed.

The question that these civilian casualty figures raise is whether they were justified. To use the criterion from Christian just war theory, were these deaths proportionate? As the Christian ethicist Nigel Biggar writes in his book, In Defence of War, this criterion holds that 'one is only justified in launching a military operation or persisting in it if the losses to one's own side (and to non-combatants) seem, according to a reasonable estimate, necessary to gain and efficient in gaining the military advantages aimed at.'

In the case of both Operation Chastise and Operation Overlord it was believed at the time, and has been generally accepted since, that this criterion was met. The civilian casualties were unavoidable in order to gain the military advantages of disrupting German industrial production and breaking out of Normandy on the way to the liberation of north-western Europe and the invasion of Germany. Furthermore, they occurred as the result of a war that was itself necessary in order to achieve the just end of the defeat of the German armed forces and hence of Nazism.

The truth of which this history reminds us (in case we needed reminding) is that wars, even just wars, do not simply affect the military personnel involved. Throughout history, and still today, they have always involved civilian casualties, including not only the civilian dead, but also those who are wounded and those who are damaged psychologically through what they have experienced and through the loss of loved ones.

This truth came to my mind last week when I read the presidential address given by the Archbishop of Canterbury at last week's meeting of the Church of England's General Synod. In the course of this address he declared: ".... as a religious leader I can say that the killing of so many civilians, the extensive damage to civilian infrastructure cannot be morally justified.

"When I visited Jerusalem, I joined the remarkable, extraordinary Christian leaders there, united as they never have been literally in their history, in calling for an immediate humanitarian ceasefire. That was over three weeks ago. Thousands more innocent men, women and children in Gaza have been killed since then – while thousands in Israel still mourn those killed on the 7th October, and hundreds of families still plead for the release of their loved ones.

"So I repeat that call again today with renewed urgency and even more force. This bloodshed must cease, hostages must be released, and aid must reach those in Gaza in dire need.

"I do not have military or political answers to this crisis. I do not speak from those perspectives. But the call for a ceasefire is a moral cry that we are hearing from people of many faiths and none. Our common humanity must find another way to achieve justice, security and peaceful co-existence for Israelis and Palestinians from now, for the future. In Christ's name, we cry out from our hearts: "No More. The killing must stop."

The problem with what the Archbishop says in this quotation is that what he is asking for is essentially for the Israel Defence Force (IDF) to immediately stop its operations in Gaza. To understand why this is a problem we have to ask three basic questions about the actions of the IDF. Is it justified in taking military action in Gaza at all? If its activity is justified, has it yet achieved its aim? Is its activity proportionate in the sense outlined by Biggar?

From the perspective of Christian just war theory the answer to the first question has to be 'yes.' According to a Christian understanding of the role of government based on the teaching of Paul in Romans 13:1-7 the role of governments is to combat human evil, using deadly force if necessary, and among the primary evils that governments need to combat is attacks on the lives and property of a country's inhabitants.

Hamas has made it abundantly clear that, given the opportunity, they will continue to mount the same kind of attacks on Israel that took place on Oct 7 until Israel is destroyed. For example, as reported in the Israeli newspaper Haaretz: "A senior Hamas official said in an interview aired last week that the October 7 attack against Israel was just the beginning, vowing to launch 'a second, a third, a fourth' attack until the country is annihilated."

Ghazi Hamad – whose comments were transcribed by the Middle East Media Research Institute (MEMRI), a Washington-based think tank – added in a LBC interview that "Israel has no place on our land. We must remove the country because it constitutes a security, military and political catastrophe."

As for the October 7 attacks – which left more than 1,300 Israelis in the south slaughtered and hundreds dragged into the Gaza Strip – Hamad declared that "we must teach Israel a lesson, and we will do this again and again."

Given that Hamad and others are saying this, and given Hamas' previous track record of attacks on Israel, the Israeli government would actually be failing in its God given duty if it did not employ the IDF to prevent Hamas having the ability to do what they have promised (in the same way that the government of Egypt has been employing its security forces to stop the threat posed by Islamic militants in Sinai).

In answer to the second question, the fact that the IDF is continuing its operations means that it does not think that its activity has yet achieved its aim and given that Hamas has not yet laid down its arms and released its hostages, but is on the contrary still firing missiles into Israel and still attacking the IDF, it would appear that they are right to think this.

The answer to the third question depends on whether the loss of life taking place in Gaza is necessary in order to eventually achieve the military objective of making Hamas incapable of continuing to attack Israel.

No one disputes that a tragically large number of civilian casualties, including women and children, have occurred in Gaza. However, in order to prove that these deaths were not necessary one would have to show that the just objective of defeating Hamas could be achieved without these casualties taking place. It is difficult to see how this could be the case given the fact that Gaza is for the most part a densely populated urban area and that Hamas has been fighting from within the civilian population, using homes, playgrounds, schools, mosques and hospitals as bases from which to conduct its military operations.

All the evidence we have indicates that the IDF has done what it could to avoid civilian casualties, whereas Hamas has done nothing to try to protect the civilian population. Hamas has built a network of tunnels to protect its own fighters, but has provided no shelters at all to protect the civilian population, one Hamas spokesman saying that it was the UN's responsibility to protect civilians. There are indeed credible reports of Hamas snipers firing on civilians trying to flee the fighting.

All this being the case, the question that has to be asked of the Archbishop of Canterbury is exactly what he thinks should happen? Of course, in the long run there needs to be an agreed political solution to the status of Gaza and the West Bank, but while Hamas and other Islamic militant groups think that they can get what they want by fighting they have no incentive to enter into meaningful political negotiations to achieve such a solution.

The Good Friday Agreement was only achieved in Northern Ireland when the Provisional IRA was forced to accept that it could not achieve its goal of Irish unification by force and was therefore willing to stop attacking the British security forces and disarm. This is another truth that those, like the Archbishop, calling for an immediate ceasefire in Gaza have 'forgotten to remember.'

Hamas needs to be brought to the same realisation about the futility of armed action as the IRA and from a Christian perspective Israel is justified in trying to bring this about. This means that, instead of putting pressure on Israel, the Archbishop and others need to urge the international community to concentrate on putting effective pressure on Hamas. Only in this way will a just peace be possible.

Martin Davie is a lay Anglican theologian and Associate Tutor in Doctrine at Wycliffe Hall, Oxford.