The ceremonial splendour of a state visit was deployed to welcome the South Korean President Yoon Suk Yeol.
King Charles and Queen Camilla greeted the president and his wife at Horse Guards Parade in London.
The royal couple and their guests had a carriage procession along the Mall, lined with South Korean flags, before going inside Buckingham Palace.
Also meeting the South Korean leaders were PM Rishi Sunak and Lord Cameron, now returned as foreign secretary.
More than 1,000 soldiers were on parade, with gun salutes in the autumn leaves in Green Park, as the South Korean president was given a ceremonial welcome, with Prince William and Catherine part of the procession.
State visits are a "soft power" mix of pageantry and practical politics - and this was a sign of respect to an increasingly important ally and trade partner, in a region with growing tensions with China.
The jingle of the cavalry harnesses on the Mall is linked to the jingle of cash tills, with trade deals to be negotiated.
Mr Yoon is having a state banquet in the Buckingham Palace ballroom, along with 170 guests, who will hear speeches from the King and the president.
These are opulent occasions, with diplomacy fuelled by fine dining, using a 19th Century dinner service with more than 4,000 pieces.
The table settings are as precise and symmetrical as the military parade - each guest getting a place setting of 46cms.
Although there might not be a repeat of Mr Yoon's karaoke-style performance when he visited US President Joe Biden in April, and sang American Pie.
Earlier this month, the King had a taste of Korean culture and K-pop, when he visited New Malden, in south-west London, which is known as "Korea Town", for having the biggest concentration of South Koreans in Europe.
While in London, the president is launching plans for a new trade deal, including technology and green energy, and closer military ties.
A "Downing Street Accord" is being signed at a meeting between the president and Rishi Sunak, which is intended to boost trade and support "global stability".
There are plans for a stronger approach to enforcing sanctions against North Korea, and preventing its "illegal weapons programme", with joint sea patrols between the South Korean navy and the Royal Navy.
"Long term, global partnerships are vital to our prosperity and security," said Mr Sunak, who added that "close ties have already propelled £21bn of investment between our countries".