WHAT would Donald Trump do? It is a question few dare contemplate for fear of making the crisis in Ukraine more terrifying. Let it be known, though, that the man himself has been musing on events in Europe.

On a radio show the other day he described Vladimir Putin’s effective pocketing of large swathes of eastern Ukraine as “genius”, adding: “He’s gonna go in and be a peacekeeper. That’s the strongest peace force. We could use that on our southern border. That’s the strongest peace force I’ve ever seen. There were more army tanks than I’ve ever seen. They’re gonna keep peace all right. No, but think of it. Here’s a guy who’s very savvy, I know him very well. Very, very well.”

It takes you right back there, doesn’t it, to the grim times when the fate of the free world was in those tiny hands. We know Mr Trump knows the Russian President well. We just did not expect him to draw attention to it considering the criticism he faced for his closeness to Mr Putin, even, on one occasion, saying he believed the Russian leader over US intelligence agencies when it came to alleged foreign interference in the 2016 election.

Who knows how a second-term President Trump would have reacted to Russia’s invasion. For the moment such questions are mercifully the stuff of speculation. What matters is not what “45” would have done but what “46”, Joe Biden, is doing, and for the moment that is talking tough.

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In an address on Tuesday night, President Biden said Russia’s actions were “a flagrant violation of international law” that demanded a firm response, in this instance America following the EU and UK in imposing limited sanctions on the Putin regime, and some of those who benefit most from it. If Russia goes further, he warned, the US will go further on sanctions.

President Biden, at 79, has a decade on the Russian leader. At times during his address the age gap told in his delivery. In terms of clarity, though, he was head and shoulders above Mr Putin and his rambling lecture about “ancient Russian soil”.

Whenever one hears the Joe Biden of today it is hard not to recall those tapes of the then Senator Biden in conversation with David Frost in 1987. Of course the Congressman sounded young and full of vigour – he was only 45. Rare is the voice that does not grow weaker with age.

Still, here we are, more than three decades on, and Mr Biden is arguably facing the crisis for which he has spent his whole career, maybe even life, preparing. It is inevitable that friends and foes alike should wonder if he will pass the test.

If you believe that a politician’s past is a reasonable enough guide to their future then Mr Biden is a man who is hard to pin down. Hawk or dove, he has been both. He opposed Vietnam and was against the Gulf War in 1991. But he was for the invasion of Iraq in 2003, which he has since said was a mistake. As Vice-President to Barack Obama, he backed talking to Iran on a nuclear deal, opposed a troop surge in Afghanistan, and famously advised his commander-in-chief against launching the raid in which bin Laden was killed.

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Perhaps it was inevitable that over the course of such a long career Mr Biden should have shifted position several times. Yet one can see why critics wonder about his judgement at times.

But then Vice-President Biden took his cue from the boss, a President far more interested in what was happening at home than abroad. Obama was the drone President, the ultimate hands-off commander-in-chief. He did get tough over the use of chemical weapons in Syria, but what he said was a “red line” turned out not to be. Hardly the administration’s finest hour, and the wrong signal to send to dictators everywhere.

On becoming President, Mr Biden told the world that “America is back”. He signed the US up again to the Paris agreement on climate change, he vowed to strengthen Nato rather than undermine it as his predecessor had done. But then came the fall of Afghanistan to the Taliban and the chaotic withdrawal of US forces from Kabul. One newspaper called it “the worst self-inflicted humiliation in [America’s] history”. It continues to be the lowest point in the Biden presidency, albeit more internationally than domestically.

It is worth reflecting on Kabul when it comes to the thorny question of what happens next over Ukraine. Mr Biden has his own reasons for not sending American forces into harm’s way, one of them intensely personal. His son, Beau, served in the military. Mr Biden long suspected a link between Beau’s tour in Iraq, during which he came into contact with toxic material, and the brain cancer that killed him in 2015, age 46.

In a speech to military families last year, the President implored society to remember the debt owed to those who paid the ultimate price, and the families left behind. “My heart is torn in half by the grief,” he said.

That was the same Joe Biden who spoke to his fellow Americans this week, telling them: “We have no intention of fighting Russia. We want to send an unmistakable message though: that the US, together with her allies, will defend every inch of Nato territory.”

Mr Putin would doubtless say the same about every inch of Russian territory. He accuses Nato of blatant deception, of aggressively expanding eastwards with no thought to Russia’s right to protect itself. There is something in that. Not nearly enough to justify Mr Putin’s actions, but worth noting. You cannot begin to understand Russia without acknowledging its justified fear of encirclement.

While not denying countries the chance to join Nato after the collapse of communism in eastern Europe, the membership process could have been gone through at a slower pace, and with due regard to Russian sensitivities.

Just as Mr Biden has no intention of fighting Russia, so the UK Government has steered clear of promising any forces to Ukraine. This is as it should be, despite the murmurings of the armchair generals. What heat there is in this new cold war needs to be taken down several degrees. Mr Biden could be the President who makes that happen. Which Mr Biden the world gets this time, the hawk or the dove, we wait to find out.