In November 1939, Queen Elizabeth (later the Queen Mother) gave Lord Halifax, the foreign secretary, a copy of Adolf Hitler’s book Mein Kampf, warning him of the Fuhrer’s “obvious sincerity” but jokingly advising him not to read it, “or you might go mad and that would be a great pity”. In that book, Hitler set out his views on the need for Lebensraum (living space) in the east of Europe, which of course included Ukraine.

It is rare for dictators to set out their thoughts so plainly, but when they do it makes sense to listen to what they say. Last July, Vladimir Putin wrote his famous 6,885-word essay “On the Historical Unity of Russians and Ukrainians”, which explained why he believed that Ukraine did not deserve a destiny separate from Russia’s. It contained such gems as “We respect the Ukrainian language and traditions. We respect Ukrainians’ desire to see their country free, safe and prosperous”, and approvingly quoted from Oleg the Prophet saying of Kyiv, “Let it be the mother of all Russian cities”.

Today, with Kyiv the mother of all battlefields, it is worth re-reading the essay for an indication of what Putin might do next, because the monstrous events since 24 February 2022 followed on naturally from his warped but, to paraphrase the Queen Mother, obviously sincere view of Russian history.

For that essay was not just about Ukraine. The Baltic state and Nato member Lithuania was mentioned no fewer than 17 times in it, and in language not wholly dissimilar from that which he used towards Ukraine. Because of what is happening today in Ukraine, where the Russian campaign is about to restart and could easily be successful were chemical and biological weapons used or more cities razed as in Chechnya and Syria, we should pay attention to what Putin thinks about other matters.

Writing of the 13th century, Putin avers that “The southern and western Russian lands largely became part of the Grand Duchy of Lithuania, which – most significantly – was referred to in historical records as the Grand Duchy of Lithuania and Russia”. Why should that be “most significant” if Putin believes today’s Lithuania to be a separate, legitimate, non-Russian country? Of the period of the battle of Kulikovo, Putin adds that “Grand Duke of Lithuania Jogaila – son of the Princess of Tver – led his troops to join with [the warlord] Mamai. These are all pages of our shared history, reflecting its complex and multi-dimensional nature.”

The Battle of Kulikovo took place on 8 September 1380. Putin’s concentration on events of over 600 years ago (“In the 14th century, Lithuania’s ruling elite converted to Catholicism”) are akin to Boris Johnson claiming suzerainty over western France because of the battle of Poitiers. Putin further notes, with evident disdain, Lithuanian’s 1569 “Union of Lublin with the Kingdom of Poland to form the Polish–Lithuanian Commonwealth”, and that “In its 1649 appeal to the king of the Polish–Lithuanian Commonwealth, the Zaporizhian Host demanded that the rights of the Russian Orthodox population be respected”. To put that in a British context, that was the same year that King Charles I was executed. Putin’s historical resentments – real or imagined – thus go back to the era of the English Civil War, and far further.

“The principal and proper work of history,” wrote Thomas Hobbes, “[is] to instruct and enable men, by the knowledge of actions past, to bear themselves prudently in the present and providently towards the future.” It should not be to revive controversies from several centuries ago in order to create spurious reasons for invading one’s sovereign neighbours.

Yet just as he did with Ukraine, the pseudo-historian Putin tells us that in the early 17th century: “One of the hierarchs of the Uniate Church, Joseph Rutsky, communicated to Rome that people in Moscovia called Russians from the Polish-Lithuanian Commonwealth their brothers, that their written language was absolutely identical, and differences in the vernacular were insignificant.” He later concludes that: “Our spiritual unity has also been attacked” and “as in the days of the Grand Duchy of Lithuania, a new ecclesiastical has been initiated.”

Should Putin win in Ukraine – and he still enjoys a numerical preponderance in every area of weaponry and is evidently hell-bent on taking the four besieged cities – he has made it clear where he would like to go next. We need to listen to his threats. Although ethnic Russians make up only 5 per cent of Lithuania’s population, they make up 24 per cent of Estonia’s and 25 per cent of Latvia’s. Although these ethnic Russians are not separatist, Putin would not be hard-pressed to find a casus bellum.

Which is where Nato must stand absolutely firm and be totally unequivocal. President Biden cannot afford to use the same kind of language about incursions versus invasions that he so woefully did before the Russian invasion of Ukraine. This is no time to mix up the Ukrainians and Iranians, as he did in his State of the Union Address.

Article 5 of the Nato Treaty only works if potential aggressors appreciate in perfectly clear terms that one Russian boot crossing one inch of Baltic territory will be considered an attack on all of Nato, and will be met with a declaration of war.

Anthony Blinken, the US Secretary of State, has been good on this subject recently, and listening to him there is no reason to believe that the United States would not invoke Article 5, and thus be prepared to sacrifice Chicago for Vilnius, Riga or Tallinn. Yet even to put it in those terms must invite doubt, and so it cannot be reiterated too regularly or too strongly.

We are often far too clever for our own good when it comes to dealing with dictators. We frequently assume that they are merely preaching to domestic audiences, or sending coded messages, and that they do not mean it literally when Adolf Hitler said in January 1939 that the outbreak of war would “lead to the annihilation of the Jewish race in Europe”, or when today’s Iranian mullahs say they will use a nuclear bomb to “wipe Israel off the face of the earth”. Over Lithuania, we cannot say we have not been warned.

Andrew Roberts’s ‘Leadership in War’ is published by Penguin