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The extraordinary measures taken to transport Ukrainian President Volodymyr Zelensky from Ukraine to the US capital are a sign of just how crucial the relationship is for both countries.

After visiting the front line in eastern Ukraine on Tuesday, Mr Zelensky's journey to Washington DC began with an overnight train journey to Poland before boarding a US Air Force plane, reportedly supported by a Nato spy-plane and an F-15 fighter jet.

Reports of the visit to Washington began circulating early this week, but it wasn't confirmed until early on Wednesday morning, when US officials felt certain that Ukraine's leader was safely en route to the US capital.

The visit had been discussed for months but the final preparations were made quickly - the two presidents spoke about it on 11 December and an invitation was extended to Mr Zelensky three days later. Only once the visit had been confirmed could final plans be put into action.

Unsurprisingly, no official information about the journey was released - security is tight around presidential visits even during peacetime, but for a wartime leader the risks are greater still.

With the threat of Russian missiles making air travel over Ukraine too risky, Mr Zelensky appears to have taken a secretive train ride through Ukraine to Poland, where he was spotted early on Wednesday at a railway station in the border town of Przemysl.

Pictures from Polish TV showed an entourage including Mr Zelensky walking along a platform with a blue-yellow Ukrainian train in the background. The group then got into a convoy of waiting cars, including black Chevrolet Suburbans - a favourite model of the US government.

Many Western leaders and officials have travelled by train to visit Mr Zelensky in Kyiv, but this was his first time abroad since Russia's war began.

Not long afterwards, flight data showed a US Air Force Boeing C-40B - thought to be transporting Mr Zelensky - taking off from Rzeszow airport some 80km (50 miles) to the west.

The plane headed northwest towards the United Kingdom, but before it entered the airspace above the North Sea, a Nato spy-plane scanned the area. The sea is known to be patrolled by Russian submarines.

And a US F-15 fighter jet, which took off from a base in England, escorted Mr Zelensky's flight for part of its journey.

Finally, around noon in Washington - nearly 10 hours after take-off and many more hours of travel for the Ukrainian president - the plane landed near Washington.

On arrival, he was given Secret Service protection - as is the case for all visiting heads of state - but Mr Zelensky's status as leader of a country at war with Russia meant security officials were being extra-vigilant.

"We are very cognisant that Russia has assets in this country and might try to do something," one senior official told ABC News. "We know what is at stake."

Despite the concerns, the visit appeared to pass smoothly. On Thursday he stopped briefly in Poland on his way home to meet his counterpart, Andrzej Duda. And by Friday he was back at his desk in Kyiv, posting a short video on Telegram: "We are working towards victory," he said.