President Donald Trump is due to give a major speech in Poland's capital Warsaw, laying out his vision for America's relations with Europe.
Mr Trump can expect a warm welcome when he addresses crowds at Krasinski Square - as Poland's conservative government shares his hostility to immigration and a strong sense of national sovereignty.
And he will promote trade at talks with Central and Eastern European leaders.
The visit comes ahead of a G20 summit of world leaders in Hamburg, Germany.
Trade talks - is it all about the gas?
First on Mr Trump's busy Thursday agenda will be the Three Seas summit - a Polish and Croatian initiative bringing together 12 nations between the Baltic, the Adriatic and the Black Sea.
The US president is expected to bang the drum for US trade, telling the gathering that American gas imports could reduce their reliance on Russian energy.
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The Polish government is trying to become less energy-dependent on Russia, and last month the first ever US liquefied natural gas shipment arrived.
Warsaw and three Baltic states have also voiced their opposition to Russia's plan to build a gas pipeline to Germany.
Central Europeans do not want to extend their dependence on Russian gas, but Berlin supports the project. This has caused tensions between the so-called "old" and "new" EU members.
What about Trump's speech at Krasinski Square?
After the Three Seas summit, Mr Trump will address crowds at the monument to the 1944 Warsaw Uprising at about 1200 BST.
The president is expected to urge the EU to take inspiration from Poland's resistance to the Nazis.
He will also set out his vision for America's future relationship with Europe.
And he will reaffirm US support for Eastern Europe ahead of his first meeting with Russian President Vladimir Putin at the G20 summit, the BBC's James Landale in Warsaw reports.
Last week, Mr Trump's national security adviser HR McMaster said: "He (President Trump) will lay out a vision not only for America's future relationship with Europe, but the future of our trans-Atlantic alliance, and what that means for American security and American prosperity."
But the big question will be how strongly Mr Trump will emphasise America's commitment to Nato's mutual defence, our correspondent says.
Mr Trump once described Nato as an "obsolete" alliance, saying that many members have failed to meet the military spending target of 2% of their gross domestic product.
Tricky G20 summit
Later on Thursday, the US leader will travel to Hamburg for what could be a potentially fractious summit.
He has fallen out with some EU nations over climate change and trade - the issues that are expected to be discussed in Germany.
UK Prime Minister Theresa May will challenge President Trump over climate change, saying that an international agreement on the issue should not be renegotiated.
"She will say that we don't see any need for renegotiation - we support the Paris Agreement because it delivers," an official within the UK government said.
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Security has also been beefed up in Hamburg, where thousands of people are planning more protests to disrupt the gathering.
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