Originally Published: 10 JUL 23 00:01 ET

Updated: 10 JUL 23 12:59 ET

By Betsy Klein, CNN

London (CNN) — President Joe Biden and King Charles III on Monday met for the first time since the British monarch’s coronation, with the US president visiting Windsor Castle for all the pomp and circumstance that comes with a royal meeting.

Biden arrived to inspect an honor guard formed of the Prince of Wales Company of the Welsh Guards – with hundreds of uniformed troops, and its military band – positioned on the grassy quadrangle before a tent. The band played “God Save the King” upon the monarch’s arrival and “The Star-Spangled Banner” upon Biden’s entrance.

The moment marked Biden’s second trip to Windsor Castle since taking office – the president met the King’s mother, Queen Elizabeth II, at her home just outside London in June 2021. The Queen met 12 US presidents spanning her reign, all but President Lyndon Johnson. The president said at the time the Queen wanted to know about Russian President Vladimir Putin, whom Biden was meeting in Switzerland days after their visit, and Chinese President Xi Jinping. Biden said he wished he could have spoken to the Queen for longer. “She was very generous,” he told reporters.

This latest meeting with Charles was a closely watched moment for how the King balances his traditionally apolitical role with a cause he is passionate about that has become a signature priority. Biden has called climate change “the existential threat to human existence as we know it.”

The meeting was also closely watched for how the two men interacted. Biden at one point put his hand on King Charles’ back, which was described as a “wonderful symbol of warmth and affection” by a royal source who spoke to CNN, denying that this action by the president broke protocol. Initiating physical contact with the monarch is generally not seen as traditional in the UK.

“[His Majesty the King is] entirely comfortable with that kind of contact – and what a wonderful symbol of warmth and affection it was between both the individuals and their nations!” the source said.

“Contrary to some reports that is, in fact, the correct protocol,” they said.

Biden, King Charles and special envoy for climate John Kerry met with private sector company leaders at a climate event. The group discussed barriers to private investment, and Kerry called Monday’s engagement “time well spent,” praising King Charles’ “convening power” on a critical issue.

The attendees, he told CNN’s Max Foster in an interview following the engagement, all “agreed that we need to accelerate the deployment of capital, investment in the new energy economy.”

Biden was particularly interested in a part of the discussion focused on “what could be done in the insurance industry to be able to provide insurance for certain types of investments.” Another part of the conversation, Kerry said, focused on “getting multilateral development banks and corporations to join together to de-risk the investment.”

“There’s a lot of capital in the world. But some of it’s been sitting on the sidelines. And what we need to do now is accelerate the development of new technologies and also the deployment of existing technology: solar, wind, nuclear, and so forth,” he said.

Kerry declined to comment on King Charles’ first meeting as monarch on a policy issue, but said there was a “great discussion” and characterized it as a “briefing” for the King, who he said “didn’t take part in the meeting” itself.

The King, Kerry said, “does have extraordinary convening power.”

“He obviously has great respect for people that are pushing an issue he’s cared about for all of his life,” Kerry said of the monarch.

In keeping with US tradition, Biden did not travel to London for the coronation, but first lady Dr. Jill Biden and granddaughter Finnegan Biden attended the ceremony. Both the president and first lady did make the trip across the Atlantic for the funeral of Queen Elizabeth II last year.

Meeting with Sunak

Earlier on Monday, Biden kicked off the first full day of his trip abroad with a London visit aimed at bolstering the US-UK “special relationship” on the eve of a high-stakes summit with NATO leaders.

Biden arrived at 10 Downing Street and was greeted by Prime Minister Rishi Sunak ahead of discussions on a range of issues, including Ukraine, a topic on which the two leaders have closely coordinated. Biden recounted all of the places he’s met with Sunak – from San Diego, California, to Belfast, Northern Ireland, to Hiroshima, Japan, to Washington, DC – six times in the six months since the prime minister took office.

“Couldn’t be meeting with a closer friend or greater ally. Got a lot to talk about,” Biden said, adding, “Our relationship is rock solid. … And I look forward to our discussions.”

Sunak welcomed Biden back to 10 Downing Street, which he was visiting for the first time as president, saying he is “very privileged and fortunate to have you here.”

He said they would be strengthening cooperation on joint economic security, as well as discussing the NATO alliance.

“We head from here to NATO in Vilnius, where we stand as two of the firmest allies in that alliance and I know we want to do everything we can to strengthen Euro-Atlantic security. Great pleasure to have you here,” Sunak said.

Their meeting came after the US announced Friday that it will be sending cluster munitions to Ukraine for the first time, a rare topic on which the US and United Kingdom publicly disagree. The UK, Sunak told reporters Saturday, is “signatory to a convention which prohibits the production or use of cluster munitions and discourages their use.”

Sunak continued, “We will continue to do our part to support Ukraine against Russia’s illegal and unprovoked invasion, but we’ve done that by providing heavy battle tanks and most recently long-range weapons, and hopefully all countries can continue to support Ukraine.”

National security adviser Jake Sullivan downplayed any concern that Biden’s decision to send cluster munitions would present any “fracture” with allied countries that oppose the use of such equipment, suggesting that Sunak was stating a “legal position” as he highlighted broader US-UK unity.

“The prime minister stated the UK’s legal position, that they are a signatory to the Oslo Convention. The United States is not. That being a signatory means discouraging the use of these weapons. He fulfilled his legal obligation, but I think you will find Prime Minister Sunak and President Biden on the same page strategically on Ukraine, in lockstep on the bigger picture of what we’re trying to accomplish and as united as ever, both in this conflict and writ large,” Sullivan told reporters aboard Air Force One Sunday.

Sullivan noted that the US has not received any negative feedback from NATO allies regarding the decision.

“That will be repeated, in my view, with all the leaders of the alliance. I do not think you will see fracture, division, or disunity… as a result of this decision. Even though many allies - the signatories to Oslo - are in a position where they themselves cannot say, ‘We are for cluster munitions.’ But we have heard nothing from people saying this cast doubt on our commitment, this cast doubt on coalition unity, or this cast doubt on our belief that the United States is playing a vital and positive role as leader of this coalition in Ukraine,” he said.

A Defense Department release on the US’ latest equipment drawdown also said that the decision was made following “extensive consultations with Congress and our Allies and partners.”

In a readout following the meeting, the White House said Biden and Sunak “reviewed preparations for the upcoming NATO Summit in Vilnius.”

“They reaffirmed their steadfast support for Ukraine in the face of Russia’s ongoing aggression,” the White House said, adding the two leaders also discussed last month’s newly announced economic partnership and developments in Northern Ireland, including “efforts to ensure continued progress there.”

Later Monday, the president departs London for Vilnius, Lithuania, where NATO leaders will gather for critical meetings amid the war in Ukraine and last month’s failed coup attempt in Russia, posing the biggest threat to global stability for the alliance in recent history.

Following the NATO Summit, Biden travels to Helsinki, Finland, where he will offer a notable show of support to Nordic countries during a summit with the leaders of Finland, Sweden, Norway, Iceland and Denmark.