For some inscrutable reason, I've been reading "The Gathering Storm" recently. Not the 11th volume of the Wheel of Time. Rather, I've been reading the first volume of six by Winston Churchill on the second World War.
While I have not completed reading it yet (nor have I made any progress on This Time is Different, which if I recall correctly has some interesting statistics for that period of time), one very common theme in that volume is that the war was preventable. That was Churchill's view. Geopolitically, Germany rising again was not preventable. However, the war could have been prevented.
Another way to put it would be that Germany rebuilding itself as a powerhouse was geopolitically inevitable. Whether it would be a military powerhouse desiring war or an economic powerhouse was what could have been influenced by the UK and France, but unfortunately was not.
Keeping in mind the bias that "to understand is to believe," I think I could subscribe to Churchill's view.
I'll let some of his words speak for themselves on the preventability of that war, or if not preventability, at least being able to avoid the disastrous need to be evacuated from Dunkirk for inadequate training of the BEF and statistical inferiority of the RAF to the Luftwaffe:
While remaining sufficiently armed themselves, they must enforce with tireless vigilance and authority the clauses of the treaty which forbid the revival of their late antagonist's military power. Secondly, they should do all that is possible to reconcile the defeated nation to its lot by acts of benevolence designed to procure the greatest amount of prosperity in the beaten country, and labour by every means to create a basis of true friendship and of common interests, so that the incentive to appeal again to arms will be continually diminished. In these years I coined the maxim, "The redress of the grievances of the vanquished should precede the disarmament of the victors." As will be seen, the reverse process was, to a large extent, followed by Britain, the United States, and France. And thereby hangs the tale. [Location 798-801 in Kindle edition]
On March, 16, 1933, the MacDonald Plan was introduced, suggesting that the French army be reduced from 500,000 to 200,000 men and the German army allowed to reach parity with the French one. Churchill commented:
The Germans demand equality in weapons and equality in the organisation of armies and fleets, and we have been told, "You cannot keep so great a nation in an inferior position. What others have, they must have." I have never agreed. It is a most dangerous demand to make. Nothing in life is eternal, but as surely as Germany acquires full military equality with her neighbours while her own grievances are still unredressed and while she is in the temper which we have unhappily seen, so surely shall we see ourselves within a measurable distance of the renewal of general European war.
... One of the things which we were told after the Great War would be a security to us was that Germany would be a democracy with Parliamentary institutions. All that has been swept away. You have the most grim dictatorship. You have militarism and appeals to every form of fighting spirit, from the reintroduction of duelling in the colleges to the Minister of Education advising the plentiful use of the cane in elementary schools. You have these martial or pugnacious manifestations, and also the persecution of the Jews of which so many members have spoken.... [Location 1,313-24 Kindle edition]
[Hitler] did not even trouble to accept the quixotic offers pressed upon him. With a gesture of disdain he directed the German Government to withdraw both from the Conference and from the League of Nations. Such was the fate of the MacDonald plan. [Location 1,388 Kindle edition]
All this time [up until 1934] the Allies possessed the strength, and the right, to prevent any visible or tangible German rearmament, and Germany must have obeyed a strong united demand from Britain, France, and Italy to bring her actions into conformity with what the Peace Treaties had prescribed. In reviewing again the history of the eight years from 1930 to 1938 we can see how much time we had. Up till 1934 at least German rearmament could have been prevented without the loss of a single life. It was not time that was lacking. [Location 933 Kindle edition]
On March 14, 1933, a statement of Churchill's:
I regretted to hear the Under-Secretary say that we were only the fifth Air Power, and that the ten-year programme was suspended for another year. I was sorry to hear him boast that the Air Ministry had not laid down a single new unit this year. All these ideas are being increasingly stultified by the march of events, and we should be well advised to concentrate upon our air defences with greater vigour. [Location 1266 in Kindle edition]
Mr. Baldwin, on Nov 28, 1934, refuting a statement by Churchill on German air force rearmament:
It is not the case that Germany is rapidly approaching equality with us. I pointed out that the German figures are total figures, not first-line strength figures [...] Germany is actively engaged in the production of service aircraft, but her real strength is not 50 per cent of our strength in Europe to-day. As for the position this time next year, if she continues to execute her air programme without acceleration, and if we continue to carry out at the present approved rate the expansion announced to Parliament in July, so far from the German military Air Force being at least as strong as, and probably stronger than, our own, we estimate that we shall still have a margin in Europe alone of nearly 50 per cent. [Locations 2,032-42 in Kindle edition]
And yet, either the Air Ministry had poor forecasters or intelligence about Germany (people in general are poor forecasters though), for:
[A]t the end of March the Foreign Secretary and Mr. Eden paid a visit to Herr Hitler in Germany, and in the course of an important conversation, the text of which is on record, they were told personally by him that the German Air Force had already reached parity with Great Britain. [...] We had indeed fallen into an ambush. [Locations 2064-68 in Kindle edition]
Henceforward all the unknown, immeasurable threats which overhung London from air attack would be a definite and compelling factor in all our decisions. Moreover, we could never catch up; or at any rate the Government never did catch up. Credit is due to them and the Air Ministry for the high efficiency of the Royal Air Force. But the pledge that air parity would be maintained was irretrievably broken. [...] Very considerable efforts were made by the British Government in the next four years, and there is no doubt that we excelled in air quality; but quantity was henceforth beyond us. The outbreak of the war found us with barely half the German numbers. [Locations 2,200-6 in Kindle edition]
During the Battle of Britain, the RAF took down three to four enemy planes for one British loss, which is a quantitative statement on quality, but no one in their right mind would want to enter a war against a foe with as good technology as oneself with statistically inferior numbers. And yet the British were forced to do so, in large part because of a policy of disarmament in the MacDonald-Baldwin Government which could not be rectified after 1934. If the RAF had that hit rate, but as good numbers or better than the Germans, it would be a good probability that the shortcomings of the BEF in 1940 could have been overcome.
Or maybe not. For in deployment in Europe in 1939, its commanders had frittered away the months that passed between the declaration of war (over Hitler's invasion of Poland) and the arrival of blitzkrieg close to the Channel and Britain itself. [...] The 3rd Division's commander, Major General Bernard Montgomery, had not been idle, though. His formation had been honed during five major exercises that had emphasized all of the operations it was about to carry out.[...] As events unfolded, Montgomery's staff were struck by his prescience, one reflecting 'Although Monty made no predictions about the course of the battle, it followed almost exactly the way he had anticipated in his exercises.' Unfortunately for the army, the 3rd Division was the only part of the BEF that had rehearsed to this degree. [Mark Urban, Generals, p. 269]
Even if the RAF had statistical parity or superiority to the Luftwaffe, the BEF's apparent lack of professionalism and practice would probably have resulted in a land rout, and hence an overall rout. It just may not have been as hideous and dire as it was during Operation Dynamo, the evacuation of Dunkirk.
What would the world be like if Churchill, instead of being an ignored MP, was listened to and such prevention was effected? Would we never have had a second World War? Would Britain and France have been able to hold their own against Germany, effecting a repeat of the first World War with German advances being halted in France but not pushed back? Would Dunkirk still have happened, but with fewer casualties and little to none of the war materiel abandoned on the beaches because of enough RAF fighters to repel the Luftwaffe's bombing runs and allow the evacuation to occur in a less haphazard manner? Who knows? But we can guess, and at the minimum, "The Gathering Storm" to me is a persuasive account of the tragedy of lack of prevention.