In previous calls, Zelensky had indicated to Biden and other senior officials that the United States was the first country he wanted to visit when the time was right for him to travel, according to a US official with knowledge of the conversations.

So in a December 11 phone call between the two leaders, Biden reiterated the invitation.

This time, Zelensky told Biden, was the right time.

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“I really wanted to come earlier. Mr President knows about it, but I couldn’t do it because the situation was so difficult,” Zelensky said from the Oval Office on Wednesday. The trip could happen now, the Ukrainian leader said, because “we controlled the situation and … first of all, because of your support”.

The behind-the-scenes details of Zelensky’s surprise visit to Washington were described by an aide to Pelosi, a US official and a senior administration official, all of whom requested anonymity to describe planning for the secret trip.

Once the wheels of planning started to roll, Zelensky’s 10-hour visit – which packed in an Oval Office meeting with Biden, a joint news conference at the White House and an address to a largely supportive Congress – came together quickly.

After the two presidents discussed the trip on December 11, the White House extended a formal invite on December 14, according to the senior administration official.

Zelensky accepted the invitation on Friday, and the visit was confirmed two days later, at which point White House officials notified Pelosi’s office of the Ukrainian leader’s travel plans.

“I’m delighted you were able to make the trip to be here,” Biden told Zelensky on Wednesday. “It’s an honour to be by your side in united defence against what is a brutal, brutal war being waged by Putin.”

What came about Wednesday was an elaborately executed plan by US and Ukrainian officials to swiftly and safely route Zelensky to Washington, his first known trip outside the country’s borders since Russia’s invasion in February.

The Ukrainian president crossed into Poland early Wednesday, according to Poland’s private broadcaster, TVN24, arriving at a railway station in Przemysl, a border town and the arrival point for many refugees fleeing the war.

Accompanied by the US ambassador to Ukraine, Bridget Brink, Zelensky was transported in a US embassy vehicle to an airport in Rzeszow, where he boarded a nonstop flight that landed at Joint Base Andrews shortly after noon Wednesday.

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Carrying Zelensky to Andrews was a US Air Force jet – a government plane typically used for Cabinet secretaries and other dignitaries below the president and vice-president.

The White House didn’t publicly announce the impending Zelensky visit until 1am Wednesday – waiting until they felt Zelensky was safely out of Ukraine.

Once Zelensky landed, Secret Service protection kicked in, as is typically done for visiting heads of state.

The senior administration official said the US consulted closely with Zelensky on his security, and that the Ukrainian president felt it was sufficient for him to briefly travel to the United States.

Meanwhile, Pelosi, who had travelled the world in recent months in a ferocious defence of Ukraine, had also been planting the seeds for months for a Zelensky address to Congress.

She had been at the Zagreb summit in October at the invitation of Zelensky and Ruslan Stefanchuk, the speaker of the Ukrainian parliament.

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There, Zelensky spoke to the audience of “the importance of the free world’s unshakeable solidarity with Ukraine” – an address that Pelosi emphasised in her invitation to the Ukrainian president.

The US House speaker returned from Croatia and began discussing the idea of a Zelensky address, informing the other main congressional leaders – Senate Majority Leader Chuck Schumer, House Minority Leader Kevin McCarthy, and Senate Minority Leader Mitch McConnell, – about her conversations abroad and asking for their support for the Ukrainian leader to come to the Capitol.

On Wednesday, Pelosi – just days away from handing over her gavel to Republican control – finally welcomed Zelensky to the Capitol, which she called a “profound privilege” and a “great pride”, coming at a moment when Capitol Hill is about to greenlight an additional US$45 billion of emergency aid to Ukraine.

“Your visit comes as the Congress prepares to again pass another consequential round of security, economic and humanitarian assistance,” Pelosi told Zelensky. “Within the next 48 hours, hopefully this will be done.”

Before he left Ukraine, there were clues in Zelensky’s own words that a surprise trip abroad could be in the works.

In a visit Tuesday to Bakhmut, located in Ukraine’s contested Donetsk province, Zelensky was handed a Ukrainian flag. He pledged then that he would pass on the flag “from the boys to the Congress, to the president of the United States”.

Standing before the US Congress on Wednesday night, Zelensky finally produced the flag – covered in signatures by Ukrainian troops battling on the front lines.

“They asked me to bring this flag to you, to the US Congress, to members of the House of Representatives and senators whose decisions can save millions of people,” Zelensky said in his final words to lawmakers. “So let these decisions be taken. Let this flag stay with you. Ladies and gentlemen, this flag is a symbol of our victory in this war.”