Joe Biden has arrived in the UK ahead of this weekend's G7 summit – bringing a warning for the UK not to "imperil" peace in Northern Ireland.
The US President touched down at RAF Mildenhall in Suffolk to visit US troops stationed there, before heading to Newquay late last night ahead of the summit.
He's expected to use the visit – his first overseas trip since taking office in January – to warn Boris Johnson and the European Union not to "imperil" the Northern Ireland peace process as they struggle to resolve a trade row.
His national security advisor Jake Sullivan warned that Mr Biden harbours "very deep" concerns on the issue provoked by Brexit, before he on Wednesday left for the UK, where he will attend the G7 Summit.
Aboard Air Force One, Donald Trump's successor headed from Joint Base Andrews near Washington towards RAF Mildenhall in Suffolk, ahead of a meeting with the Prime Minister today.
Joe Biden arrives in Sussex ahead of the G7 summit
Mr Sullivan said the president believes the post-Brexit Northern Ireland Protocol is "critical" to ensuring that the Good Friday Agreement is protected, as Britain and the EU try to resolve the issue of checks in the Irish Sea.
"But whatever way they find to proceed must, at its core, fundamentally protect the gains of the Good Friday Agreement and not imperil that," he added.
"And that is the message that President Biden will send when he is in Cornwall."
Mr Sullivan said the US had been "crystal clear" on the need for the Good Friday Agreement.
He told reporters: "President Biden has been crystal clear about his rock-solid belief in the Good Friday Agreement as the foundation for peaceful coexistence in Northern Ireland.
President Joe Biden delivers remarks to U.S. Air Force personnel and their families
"That agreement must be protected and any steps that imperil or undermine it will not be welcomed by the United States."
After being asked if Mr Johnson was "taking steps to imperil" the agreement, he added: "I'm not going to characterise that at this point. I'm only going to say that President Biden is going to make statements in principle on this front.
"He's not issuing threats or ultimatums, he's going to simply convey his deep-seated belief that we need to stand behind and protect this protocol."
Mr Biden flew in to the UK on Air Force One, landing at the US airbase at RAF Mildenhall.
He told an audience of US military personnel and service families: "We are going to make it clear that the United States is back and democracies of the world are standing together to tackle the toughest challenges and the issues that matter most to our future."
He called for international action to tackle climate change and end the Covid-19 pandemic.
Joe Biden at RAF Mildenhall
"This diplomacy is essential because no single nation acting alone can meet all the challenges we face today because the world is changing," Mr Biden said.
The president will then have face-to-face talks with the Prime Minister on Thursday in Cornwall, with the summit beginning in Carbis Bay the following day.
Having succeeded Mr Trump after his single fractious term in the White House, Mr Biden said the trip is about "realising America's renewed commitment to our allies and partners" in an article for the Washington Post.
He and the First Lady will travel to Cornwall later
Mr Biden also said he would meet the Prime Minister to "affirm the special relationship between our nations".
However, this week it emerged Mr Johnson is not a fan of the expression, after an American magazine reported he told aides he does not like the seemingly "needy and weak" term.
Downing Street said he "prefers not to use the phrase" and is instead intent on fostering a "close relationship" with Washington.
Part of that, Mr Johnson hopes, is a trade deal with the US.
President Biden and First Lady Jill as they arrived at Cornwall Airport in Newquay this evening
AFP via Getty Images)
However, the national security adviser's remarks were the latest in a string of warnings about Brexit's impact on Northern Ireland.
The Prime Minister plans to use the summit of leading economies to urge the members – also including Canada, Japan, France, Germany and Italy – to "defeat" Covid-19 by helping to vaccinate the world by the end of next year.
Mr Johnson also wants a new treaty on responding to pandemics after the "pretty scratchy period" where countries were competing and "squabbling" over access to personal protective equipment.
The summit would focus on "building back greener, building back better" after the pandemic, he said.
After the summit ends, the president and first lady will meet the Queen at Windsor Castle.
Mr Biden will then depart for Brussels and later Geneva in Switzerland.
His trip to Europe also takes in a Nato meeting and talks with Russia's Vladimir Putin "to let him know what I want him to know".