Mentioning Hitler in an argument is usually dangerous but sometimes justified, provided the purpose is not simply to smear an opponent. Here are examples of each and a comparative case from history concerning a common currency.
Adolph Hitler is rightly regarded as the epitome of evil so it is dangerous to mention his name in any political debate, as both London’s ex-Mayors have recently done. In Boris Johnson’s case the critical storm is preposterous. The context in which the Fuhrer was “dragged into” the argument was reasonable, if risky. Boris recalled past attempts to unify Europe undemocratically and explicitly stated that the EU’s method was different from Napoleon’s and Hitler’s – they were attempting it by war. He said, “The EU is different, it’s trying to do it in a more bureaucratic way … the EU is a peaceful organisation … operating by stealth.” It seems a fair distinction – he was trying to say that unification without democracy has never worked, regardless of the means.
Boris had in mind Nazi plans to impose on a conquered Europe a common currency, etc. In fact Boris got in his defence to Lord Heseltine’s (and others’) fulminations very early – 14 years ago in fact:
Ken Livingston’s reference to Hitler appears to be more sinister. He 
claimed that Hitler wanted to relocate Germany’s Jews to Palestine (we haven’t checked) and that therefore Hitler was a Zionist. What was his purpose in saying this? Was it: Hitler was a fascist, Hitler was a Zionist, therefore Zionists are fascists”? We don’t know, his defence is that he was “simply” stating a historical fact. It’s not that simple Ken.
If you want to know more about Germany’s wartime plans for a unified Europe consider a speech in 1940 by Walther Funk, the Reich Economic Minister:
http://www4.dr-rath-foundation.org/brussels_eu/roots/06_economic_reorganization_europe.html
“The price level will have to be adjusted to that of Germany. But a currency union will bring about a gradual levelling of living standards.
“As yet there is no hard and fast plan in existence, but only preparations for comprehensive planning in accordance with orders from Field marshal Hermann Göring, who will decide upon the final form and execution of the plans. Thus I must limit myself to a statement of the basic principles and methods. I shall therefore merely indicate the means which can be used to achieve our aim.
“The Greater European economy: whatever truth there may be in this notion, one must recognize first of all that this Greater Europe does not yet actually exist, that it has first to be created and that within its area there is still much friction.
“Obviously the war will have far-reaching effects on European as well as on world economy. We shall co-operate closely with our ally Italy in all spheres and combine German and Italian economic forces for the purpose of European reconstruction.
“The question of the future general economic order in Europe must therefore be answered as follows: after the victorious conclusion of the war we shall apply those methods in economic policy which won us our great economic successes before the war and especially in time of war, and we do not contemplate again allowing the operation of the unregulated play of forces which involved the German economy in very great difficulties. We are convinced that our methods will prove to be of great advantage not only to the Greater German economy, but also to all European economies which naturally stand in close trade relations with Germany.
“Regarding the question of the basis of a new currency … It stands to reason that the Reichsmark will have a dominant place.
“The price level will have to be adjusted to that of Germany. But a currency union will bring about a gradual levelling of living standards.
“We shall consider it important to trade our high-quality industrial products in exchange for raw materials in the world markets.
“Germany will emerge at the end of a victorious war with a potential volume of exports such as she has never had before.
“The coming peace-time economy must guarantee for Greater Germany a maximum of economic security and for the German nation a maximum consumption of goods to raise the level of the nation’s well-being. The European economy must be adapted to achieve this aim. Development will proceed by stages and differently in different countries; it is still beset with numerous uncertainties, for – we must never forget – we are still at war.”
And finally, a comment from John Longworth, who was sacked as Director General of the CBI for expressing a personal view that Britain should leave the EU (Daily Telegraph, 18th May 2016):
“In 1940, at that moment when our nation was in most jeopardy from a European “project” centred on Germany, the Conservative party was split.
The Prime Minister, Neville Chamberlain, and the foreign secretary, tipped to be prime minister, Lord Halifax, both thought it economically rational to make peace with Germany in order to keep our national wealth and to maintain the British world system of finance and trade.
By contrast, someone whom they and many in the Conservative party thought of as a maverick, a “swivel-eyed loony”, namely Winston Churchill, thought differently. He persuaded the Cabinet that once a country capitulates, it never recovers. And the rest, as they say, is history. Except here we are again.”
This EU Won't Work 