This has always been a good topic to teach because the differences of opinion mean that there is a lot of historiography for students to get their teeth into. To answer your question – a case can be made for all three perspectives that you propose: there is evidence both that it was necessary and that it was a war crime and that it was to some extent both. I shall try to provide some primary and secondary evidence and allow you to make up your own mind, something I try to do with my students.

To examine the idea that Strategic Bombing was a war crime, we must first understand what we mean by this term. The Nazi war criminals were judged under the ‘Nuremberg Principles’ which were created by the International Law Commission of the United Nations to create a definition of war crimes prior to the Nuremberg Trials. The relevant section being Principle VI which states:

Violations of the laws or customs of war which include, but are not limited to, murder, ill-treatment or deportation to slave labour or for any other purpose of civilian population of or in occupied territory; murder or ill-treatment of prisoners of war or persons on the Seas, killing of hostages, plunder of public or private property, wanton destruction of cities, towns, or villages, or devastation not justified by military necessity.

Thus we can say that bombing that is not militarily justified is a war crime. So if the strategic bombing campaign was not justified by necessity then the Allied Commanders who approved the campaign are, by definition, war criminals.

This inevitably leads to the question – what were the justifications for the bombing campaign? To my mind there are three primary ones – degradation of German industry, destruction of German infrastructure and undermining of German morale. There are also two secondary justifications – forcing the Germans to withdraw resources from the frontline to defend Germany and the British desire to actively engage the Germans in the period prior D-Day when Stalin repeatedly called for the opening of a Second Front.

The impact on German industry has been much debated – Hastings in his very readable book on Bomber Command suggests that ‘vastly more weapons would have been built – with severe consequences for the Allied armies – if factory operations had been unimpeded’. A conclusion that the excellent Adam Tooze agrees with in his assessment of bombing on coal and steel production. Tooze actually goes as far as to suggest that more strategic bombing of the Ruhr Valley could actually have won the war. There is also evidence to suggest that the American tactic of targeting synthetic oil production after the Germans lost access to Romanian oil fields yielded significant benefits. However, this has to be balanced against the significant cost of bombing to the Allies. Building and crewing aircraft with the capabilities to destroy cities is a huge undertaking, although this is slightly outside the scope of the question. However it is worth considering the huge costs in men and resources that the Allies had to pay to bomb Germany. These resources could perhaps have been much better spent elsewhere.

The effect on German morale is even more controversial. It should perhaps have been clearer to the British that large scale bombing would not break the spirits of a nation as they had endured the Blitz in the winter of 1940. Churchill himself had also stated in 1917 in response to German raids:

It is improbable that any terrorisation of the civil population which could be achieved by air attack would compel the Government of a great nation to surrender. In our own case, we have seen the combative spirit of the people roused, and not quelled, by the German air raids. Nothing we have learned of the capacity of the German population to endure suffering justifies us in assuming that they could be cowed into submission by such methods, or indeed that they would not be rendered more desperately resolved by them.

Obviously this was in response to an entirely different type of bombing but as it turns out, Churchill’s conclusions of 1917 proved to be rather prescient.

The most damning evidence against the Allied decision to bomb civilians is the fact that the War Office had sent a memorandum to the various RAF commanders at the start of the war stating: “The intentional bombardment of civil populations as such is illegal.” This assessment was echoed in the House of Lords in 1944 by the Bishop of Chichester, George Bell: “Area bombing which is the issue to-day—is definitely designed to diminish the sacrifice of British lives and to shorten the war. We all wish with all our hearts that these two objects could be achieved, but to justify methods inhumane in themselves by arguments of expediency smacks of the Nazi philosophy that Might is Right. What we do in war—which, after all, lasts a comparatively short time—affects the whole character of peace, which covers a much longer period.” The extent of the debate can be demonstrated by contrasting this view with the thoughts of Max Hastings at the end of his excellent work All Hell Let Loose:

Hitler’s people inflicted appalling sufferings upon the innocent. The destruction of their cities and the deaths of significant numbers of their inhabitants seems a price they had to pay for the horrors they unleashed upon Western civilisation, and represents a far lighter toll than Germany imposed upon the rest of Europe.

Arthur Harris, the head of RAF Bomber Command wrote after the war: “In spite of all that happened at Hamburg, bombing proved a comparatively humane method. For one thing, it saved the flower of youth of this country and of our Allies from being mown down by the military in the field, as it was in Flanders in the war of 1914-1918.”

Again, we can contrast this with the views of Kurt Vonnegut (a fantastic author who was captured by the Germans and was present in Dresden during the fateful attack): “I mourned the destruction of Dresden because it was only temporarily a Nazi city, and had for centuries been an art treasure belonging to earthlings everywhere. It could have been that again.”

Hopefully this is to some extent helpful. There are a multitude of arguments that can be made about this topic. The only thing I would caution against is the modern tendency to dismiss men like Churchill and Harris as war criminals without any attempt to place yourself in their shoes and consider the awful choices they were forced to make.

If there are any ambiguities, please ask for clarification – I am more than happy to provide extra details.