When Prince Charles compared Vladimir Putin to Adolf Hitler, in a private conversation in Canada this week, it was far from a first. Hillary Clinton has made that allusion, as have Mitt Romney, John McCain and his fellow US senator Marco Rubio.
Hitler has become the archetype for an evil dictator with global ambitions who, if he is not stopped early in his career, will trigger a world war. And the parallels with Putin are tempting. Putin also peddles ethnic nationalism, projects his power across the globe and governs his country with an iron fist.
Failure to stop him in the Ukraine could, the argument goes, be our Munich moment of unwise appeasement.
But is the comparison accurate? On the contrary, it is not only flawed but is a troubling blend of history and politics that could lead us to make some bad decisions about containing Putin's ambitions. Beware policy based on rhetorical cliches.
Historians are often nervous of comparing one figure with another when they are divided by so many years in between. The actions of historical figures are dictated by a specific context that is never identically replicated. Hitler was driven by a mix of wounded German pride, biological racism, a territorial imperative and a conviction that Bolshevism and Judaism were the same and had to be eliminated - a blend of prejudices unique to the 1930s.
Each conquest he attempted was not an end in itself but a step towards a final confrontation with Marxism and the domination of the world by the German master race.
How does Putin compare? Thinly. It is true that he uses the presence of his ethnic groups in other countries as an excuse to cross the border and grab large tracts of land. Hitler, likewise, used the supposed oppression of Sudetenland Germans as an excuse to carve up Czechoslovakia.
Yet Putin is not a totalitarian dictator on Hitler's scale. He is an autocrat who bends the rules to suit his needs and doesn't have to worry about elections. But it is generally thought that he has to command the popular will to remain in power and it is conceivable that he could lose his authority and be kicked out.
Russian journalist Leonid Bershidsky wittily observed that under the former KGB agent, Russia had evolved from a "fake democracy" to a "faux dictatorship". Yes, there are laws against unsanctioned rallies, the promotion of homosexuality or websites with "extremist content", but they are randomly enforced. For the record, homosexuality is legal in Russia, as is the right for transsexuals to change their legal gender; such liberalities were not present in Nazi Germany.
Putin does not teach his people that they have some godly duty to rule the world. On the contrary, he roots his foreign policy adventures in a language of human rights cynically borrowed from the West. Opposing Ukrainian nationalism, aspects of which have pro-Nazi sympathies, he insisted that he was resisting anti-Semitism.
Rather than lumping authoritarians together, presuming that they all share the same motivations and behave the same way, it's more helpful to consider Putin in his own historical context. Hitler's ethnic nationalism led him to believe that he had to take on the whole world; Putin wants to dominate in a limited Russian sphere of influence.
It is strange that we don't hear more discussion about the traditional Russian ambitions that led to the slaughter of the Crimean War (1853-56), or the militant pan-Slavism that emerged thereafter with a promise of uniting the Slavic peoples under the banner of Russian autocracy, which contributed to Russian entry into World War I in 1914.
But Putin's ambition, although doubtless based on romantic imperialism, is narrower than even that, being concerned simply with reuniting ethnic Russians who woke up after the fall of communism to find themselves living in new states dominated by other ethnicities.
Such is the strength of feeling among the dislocated that some have formed internationally unrecognised "buffer" states between their official homes and Russia, trying to maintain the fiction that they are still tied to Moscow.
Transnistria, lodged between Moldova and Ukraine, is a Soviet theme park of 550,000 people where the hammer and sickle hangs on walls, candidates for office occasionally garner more than 100 per cent of the vote, and gunrunning makes a substantial contribution to the GDP.
While Hitler worked to a fast timetable, conquering two countries in one year before invading Poland, Putin's foreign policy is reactive and plodding. He has generally exploited crises in former members of the Soviet bloc rather than started them and has probably not always been the master of events.
Did he want Crimea to join Russia? Almost certainly. But his determination to carve up east Ukraine is less certain. When he called for ethnic Russians to delay repatriation referendums, it was perhaps recognition that Russian claims to those areas are less solid and acceptance that Russia can only go as far as the West will allow it before the economy is crippled with sanctions.
Here is one more important difference with Hitler. Nazi Germany was a country in the ascendant, whereas Russia has a falling population, a fragile economy and a global status shrinking in comparison to newcomers such as China and India.
If Western observers are determined to make a comparison with anyone, the more accurate one would be between Putin and Slobodan Milosevic, former leader of Serbia.
Like modern Russia, Serbia was the product of the death of communism, a state that lost its regional hegemony and tried to reclaim it through political subjugation justified by protecting Serbian ethnic minorities.
Like Putin, Milosevic sometimes acted boldly, sometimes with conciliation, had his hand forced by nationalist forces beyond his control, and spoke simultaneously of uniting his brethren and protecting the wider cause of human rights. As we might suppose in the case of Putin, his nationalism was less a rigid ideology than it was a political tool. Without nationalism, Milosevic was simply an inept manager of a failing state. He finally fell from office not because he was driven out by the West but because his people were sick of the corruption and inefficiency of his government.
None of this is to suggest that because Putin is not Hitler, he is not a problem for the West. Many of the countries that fall within what he regards as his sphere of influence have become Nato members, which means Nato has an obligation to protect them. Its freedom to do that is checked by the fact that Russia remains a nuclear power, which means the tensions could be resolved not through a long, heroic world war - a la 1939 - but with a nuclear exchange that reduces the world to ashes.
And it is partly because the stakes are so high that the Hitler/Putin comparison is quite risky. It implies that anyone who urges caution is tolerating a situation that could end in genocide, putting pressure on statesmen to act in far bolder a way than is strictly necessary.
Better to try to understand Putin on his own terms and to contain his ambitions in a proportionate manner. For all its bluster, this is largely what the West has chosen to do.