Even seasoned observers of geopolitics were taken by surprise.

But behind the scenes, months of nail-bitingly tense negotiations, near misses and fraught moral compromises had been playing out in the run-up to the largest East-West prisoner swap for decades.

Not since the Cold War, when secret agents and political prisoners would be routinely exchanged on Berlin’s Bridge of Spies, have so many captives been traded.

The remarkably complex deal, which saw 24 inmates freed from prisons in seven countries, had previously collapsed and was in doubt until just before it was sealed. Among those released were two captives holding British citizenship, eight Russian agents, four Americans, five Germans and five Russian anti-Putin activists.

In the twilight hours of his presidency, embattled Joe Biden played the pacifier as Western officials bickered over terms.

In particular, the Germans were reluctant to release Kremlin assassin Vadim Krasikov – who shot dead a Chechen exile in Berlin five years ago – reportedly fearing any swap would become ‘business model’ for Russia.

But in hush-hush meetings on the fringes of other diplomatic gatherings, President Biden eventually convinced Chancellor Olaf Scholz to agree to it.

According to analysts, this was the moral compromise – and it could prove to have lasting implications. 

Because if Vladimir Putin was previously unsure about whether he could recover valued henchmen, he no longer harbours any doubts on that score.

It was a backstairs deal, if ever there was one. Due to Russia’s illegal invasion and occupation of Ukraine, East-West relations are at their worst for decades.

Yet as this exchanged proved, spymasters in London, Washington and Moscow are still talking to each other. 

Throughout the negotiations, shadowy officials set up a special channel to discuss the various prisoner proposals.

Among the detainees discussed was former US Marine Paul Whelan, a citizen of four countries, including Britain. 

He had been visiting Moscow to attend a wedding in December 2018 when he was arrested by balaclava-wearing Kremlin hoods who accused him of espionage.

Mr Whelan spent 2,043 days in custody, singing national anthems to raise his spirits and was known to joke that the Russians thought they had arrested James Bond when actually they had detained Mr Bean.

His ordeal finally ended at a quarter to midnight local time when a plane carrying him, Wall Street Journal correspondent Evan Gershkovich and others landed at Andrews Air Force Base in Maryland. 

Mr Whelan was the first to be greeted by Mr Biden and Vice-President Kamala Harris. He saluted before shaking their hands.

Months earlier it looked as if the deal had collapsed following the death of Putin critic and political rival Alexei Navalny, who perished in a Russian prison cell in February. 

Previously, Germany had been willing to trade Krasikov to secure Mr Navalny’s safety. But when he died, many observers thought the exchange would expire with him.

Work to resuscitate the deal began immediately after his death at the Munich security conference – and continued even as Mr Biden undertook his disastrous TV debate against Donald Trump.

On July 19, a Russian court found Mr Gershkovich guilty of spying and sentenced him to 16 years in a top-security detention centre. To the outside world, it had all the appearances of a devastating blow to the journalist.

But those really in the know suspected otherwise. Prior to prisoner swaps, the Moscow authorities traditionally sentence to crushingly long prison terms those they are actually soon to release.

Mr Gershkovich and the others received pardons yesterday as they finally tasted freedom.