WASHINGTON -- In the four years he served as the United Kingdom's ambassador to the United States, Sir Peter Westmacott stored up a lot of positive memories.
The one that comes to mind first happened early in his tenure -- March 2012.
"I had a very successful, happy visit by my prime minister," Westmacott recalled in an interview late last year with The Huffington Post. He said he still remembers how the magnolia trees on the South Lawn of the White House looked the day Prime Minister David Cameron arrived. What followed wasn't easily forgotten either.
"It was 500 people for a banquet, and we were privately entertained by the White House and so on," said Westmacott, who retired in mid-January.
There were other highlights, too, for the ambassador -- some more light-hearted like Prince Harry's visit in 2013, others that speak to the true value of diplomats.
Among them is the small matter of helping convince Congress to support the nuclear agreement with Iran -- or at least not scuttle it.
Westmacott and the ambassadors from France, Germany, Russia and China -- a group that together represented the five other nations that negotiated the nuclear deal alongside the U.S. -- met with senators this past August as congressional legislators were still deciding what they would do. The foreign envoys explained that the agreement was about more than the troubled U.S.-Iran relationship.
"This was about future regional security and a deal which we had all signed on to, and which we all believed was the best available, and which was going to collapse in the event that the United States killed it or voted against it," said Westmacott, recalling his argument.
After the presentation, Senate aides told HuffPost that the diplomats' remarks helped spread the view that the agreement was the best deal possible and that it would be almost impossible to gather another international coalition to press Iran for more concessions if this effort failed.
"I think that made a certain impact, and we were able to show that this was the least bad way of stopping Iran getting nuclear weapons and that 15 years' security was better than just three months," Westmacott said, comparing the length of time during which the deal will constrain Iran's uranium enrichment to how long it might have taken an unconstrained Iran to build a nuclear weapon.
The congressional effort to quash the deal died in the Senate about a month later. Earlier this month, sanctions against Iran were lifted under the agreement as Iran has complied with initial obligations.
Westmacott's successor, Sir Kim Darroch, is likely to be at least as influential around Washington because of his ties to the prime minister. He arrives after three years as Cameron's top national security adviser.
The outgoing ambassador said that during his time in Washington, he gained a deeper understanding of the strength of the U.K.-U.S. bond. He pointed both to what Americans around the country have told him and to such individual links as all the Britons who work for U.S. companies and vice versa, and all the students from each country who choose to study in the other.
He suggested the security ties will grow even stronger, now that Britain is investing more in military equipment and counter-terror efforts.
Westmacott also predicted closer cooperation because of a massive U.S.-European Union deal called the Transatlantic Trade and Investment Partnership (TTIP). Though critics have blasted the negotiations around the pending deal as too secretive, he argued that it would help keep the West competitive and connected, and even create jobs.
The next round of talks on TTIP are set for February, and Westmacott sounded a carefully optimistic note.
"We haven't yet got there -- there's TPA but there isn't yet TTIP," he said, using the acronym for Trade Promotion Authority, which the Obama administration secured from Congress last year and which makes it easier to move trade deals through Congress.
"I am very hopeful that, if not before the end of my time, at least until the end of this administration that the back of those negotiations will be broken," Westmacott said.
Watch his full remarks in the video above.
This story is part of the third installment in The Huffington Post's "Diplochats" series, which interviews prominent diplomats on important global issues. (Note: The series was previously known as "Ambassadors Unplugged." Past installments can be found here.)