It has taken a long time for western politicians to recognise the extent and depth of the threat represented by Vladimir Putin’s Russia. Some in the Labour party still don’t. It is also plain, as Theresa May embarks on an open-ended confrontation with Moscow, that the dispute provoked by the Salisbury outrage could take years to resolve.

Cold or hot, overt or covert, this is going to be a long war – and Britain will need all its friends and allies if it is to prevail against a ruthless opponent. Whether sincere, sufficient and timely support will be forthcoming is in serious doubt.

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It is not just about Salisbury, of course. Putin has been crossing red lines, at home and abroad, with growing impunity since he first gained national prominence in 1999. He made his name with a brutal pacification campaign in Chechnya justified by a series of suspicious apartment bombings. Alexander Litvinenko, later murdered in London, blamed the bombings on the FSB and, by implication, Putin.

Justified perceptions of western weakness, ambivalence and division have since encouraged Putin in a pattern of escalating, aggressive behaviour. Its main features include wars in Georgia and Ukraine, cyber-attacks against Nato countries, election meddling and destabilisation operations, and the bloody Syrian intervention.

Putin was further emboldened by his domestic dominance, achieved through manipulation of elections, the rustication of the Duma into a rubber-stamp parliament, and the elimination, by various means, of leading opponents, critics and free media. Boris Nemtsov, a liberal reformer killed in 2015, and Anna Politkovskaya, an investigative journalist murdered in 2006, are but two names on a long list that could ultimately include Sergei and Yulia Skripal.

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Underlying Putin’s actions is a sense of Russian exceptionalism – that somehow, Russia is different and not bound by the laws and obligations of the rules-based international order introduced after 1945. His attitude is rooted in the era of the dominant Soviet superpower. But its origins run deeper. Nineteenth-century tsarist Russia both envied and aped Europe. After the 1917 revolution, it defined itself in opposition to the west. Putin has revived that tradition.

Until very recently, western leaders have been reluctant to believe the evidence of their own eyes – and their intelligence agencies. There are reasons for this myopia, not all bad. At one end of the spectrum, there is genuine dread that facing off against Putin could lead to some kind of military confrontation with an insecure, paranoid leader who boasted only this month of Russia’s fearsome nuclear weapons arsenal.

Then there are the usual strategic and diplomatic considerations: Russia is an influential actor in big international issues such as North Korea and Iran. There are important business and trade interests. And then there is sheer political complacency.

George W Bush said he could see into Putin’s soul, then discovered a black hole. Naive Barack Obama tried to “reset” relations with Russia but his plan crashed. For his part, Donald Trump is an infamous Putinophile – and nobody really knows why.

When Trump assures Theresa May of his unstinting support, she should take his words with a large pinch of salt

Trump’s persistent refusal to criticise Putin directly, whether it be over use of chemical weapons in Syria or covert campaigns to subvert US elections, suggests the Russian leader has some kind of hold over him, possibly relating to Trump’s past business dealings in Russia. Robert Mueller’s federal investigation may shed more light on this.

Germany, chronically dependent on Russian energy (almost 40% of its oil imports and 35% of its gas came from Russia in 2016), has obvious vulnerabilities in a hypothetical confrontation. France has commercial interests at stake. Many in Italy want to lift Crimea-related EU sanctions. And so on and so on.

So when Trump assures Theresa May of his unstinting support, she should take his words with a large pinch of salt. Trump talks a lot. But what he does, or does not do, matters more. So far he has given Putin a free pass. So far he has refused to enact congressional sanctions over election-meddling. So far, he has acquiesced in Russia’s illegal occupation of Crimea. So far, he has dissed and undermined Nato, Britain’s last line of defence. So far, not so good.

And when Emmanuel Macron, France’s president, speaks of “solidarity”, Britain beware. That favourite French word can mean anything or nothing. Similar scepticism must apply to expressions of support emanating from Brussels and other EU capitals. Europe’s track record on taking unified, uncompromising action in response to big international challenges is unimpressive. Just look at Syria. Europe, collectively, talks a good game. But it rarely registers on the scoreboard these days. And the sniggering man in the Kremlin knows it.

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Rallying these friends and partners into a cohesive alliance capable of facing down Putin, and forcing a step change in his behaviour, is a tall order for May. And now, uniquely, it is all the more problematic because of Brexit. To suggest that bad feeling over Britain’s unamicable departure will have no impact on future EU cooperation in such cases is delusional. Naturally, everybody agrees what happened in Salisbury is an outrage. But actually doing something practical to help out the Brexiting Brits, especially if it harms national interests, is another matter entirely.

When push comes to shove, it seems unlikely Trump and Europe will give May the full backing she desperately needs as she goes up against one of the world’s most unscrupulous and dangerous leaders. It is going to be a long war – and not only with Russia.

• Simon Tisdall is an assistant editor of the Guardian and a foreign affairs columnist