I'll have to give this more thought. It doesn't sound reasonable on the surface.
That's because I'm talking about a much longer timescale, not just WW2. WW2 was simply the end-game, for Germany. Germany was in many ways the heart of the Protestant reformation and the university centre of Europe, for centuries - that is not in dispute I wouldn't have thought, that's just a fact. Also, Britain and Germany were historical allies, against France - the Anglo-Saxons were from Germany originally so the ties between Britain and Germany run very deep, both through blood and religion. Nevertheless, by the middle of the 20th century, it had been taken over by ungodly leadership, and then destroyed - largely by their own kinsmen the British and Americans. How did Satan achieve this? Through a long series of events, of which I am by no means an expert on, but which I can see the thread of Satan's influence in.
As far as I can see from my limited perspective, Satan encouraged political dealings in Europe through the later 19th century that resulted in Britain, France, America and Russia becoming aligned against Germany and Austria, at a high "elite" level, largely unknown to and against the interests of the common citizens of the countries. For instance, the British military had made secret military alliances with France that even the parliament was unaware of - yet upheld when it came to war. Satan set the stage for WW1.
WW1 was a war between good people (on all sides) led by evil leaders (on all sides). The common people didn't even know why they were fighting, hence the "Christmas truce" on the British / German front in the first year of the war. That was because the regular British and German people held no animosity towards each other and saw each other as historical relatives, allies and friends, they didn't want to be fighting each other in the first place. German POWs would tell their British captors that they couldn't understand why they were fighting against each other, and thought the British should be fighting with them against the French! It was a pointless disaster that should never have happened - but served Satan's bloodlust.
The outcome of that war was an impoverished Germany, treated extremely unjustly at the close of the war, being blamed for the entire conflict, plundered and oppressed by the winners, and forced into severe economic hardship. In desperate times people elect firm leaders who appear willing to solve the problems they face using harsh measures if necessary. The Nazi party were elected solely because of WW1 - they never would have had an opening to get into power if the German people were not desperate enough to resort to them. They were highly successful in lifting Germany out of poverty - but ultimately turned out to be an evil leadership that led to the final destruction of Germany by outside forces. Hence:
Bonhoeffer certainly felt that the Nazi regime was one that didn't represent him as a Christian. Many other Christians felt the same.
The other outcome of WW1 was the Communist revolution in Russia, forming the USSR - an even more evil empire as discussed previously. Again, that also happened because the Russian people were in such desperate need following WW1 that their culture too was sufficiently destroyed so the people were ready for a revolutionary takeover.
This all set the stage for WW2, which in many ways was just a continuation of WW1. By the end of WW2, many Christians had been killed (on all sides). The German historical Christian sites had been bombed to oblivion. Half of Germany was under the control of the communists, who directly worked to stamp out the Church. The other half had religious freedom, but suffered much persecution and economic destruction also.
Satan had taken the core of Protestantism, perverted or killed the people and blown up their history, over about half a century of politics and war. Western Christendom was now left in the leadership of Britain and the USA - but he already had plans to pervert and destroy it there too, just differently, as we've seen subsequently.
