When the Duke of Cambridge and Duke of Sussex went head-to-head on the polo field, it was the latest in a lifetime of friendly rivalry on the sports field.
Today, for the first time, the brothers had very special cheerleaders on their sides, as their four children enjoyed a full family outing to support them.
Prince George, Princess Charlotte, Prince Louis and Archie Mountbatten-Windsor made their first public appearance together at the polo, in a rare day out that saw the Queen's great-grandchildren at play, the Telegraph UK reports.
The Duchess of Sussex cradled two-month-old Archie, kissing him on the head and carefully holding his wriggling body as she watched the polo.
Archie's older Cambridge cousins, meanwhile, seemed veterans of the polo-watching scene in comparison, with Prince George wielding his own mallet as they entertained themselves.
Prince Louis, who at one point put the Duchess of Cambridge's sunglasses on and stuck his tongue out, made his first appearance at a match, toddling confidently around the grass and pointing enthusiastically to the horses.
The family gathering, at the Billingbear Polo Club in Berkshire, was officially in aid of the Khun Vichai Srivaddhanaprabha Memorial Polo Trophy, named after the late owner of Leicester City who died in a helicopter crash in October.
The match raised more than £1 million for charities close to the Dukes' hearts, with causes including conservation, mountain rescue, and the Invictus Games.
The brothers played against one another, with the Duke of Cambridge's team eventually winning 7-6 in a spirited match which saw them captured on camera gritting their teeth as they fought for the ball on horseback.
Their tiny cheerleaders waited on the sidelines with the Duchesses, who had their hands full keeping four children entertained.
The joyous outing came after a difficult week for the Sussexes, in which they have been criticised for the secrecy around formal elements of Archie's christening and the Duchess' "private" appearance at Wimbledon.
It also came after persistent rumours of a rift between the in-laws, with their working differences leading to the split of their official households and Royal Foundation earlier this year.
The Cambridges and Sussexes had been together at the christening at Windsor Castle on Saturday, but were only seen together in an official photograph issued afterwards.
The appearance of the Duchesses and their children had not been previously announced, but the charity polo match had been publicised by the palace, with invitations issued to host of photographers and journalists to watch on from the sidelines.
Today, they appeared at ease in front of the cameras, with new mother the Duchess of Sussex concentrating on keeping a bare-footed Archie in the shade of the trees.
After the match, a sweaty Prince Harry came to visit his wife and baby son, who was by then wrapped in a blanket ready to go home.
The three Cambridge children, meanwhile, enjoyed the day out in their own way, with activities from football to a picnic out of the boot of their car.
Prince Louis stole the show, making a break from his mother to take a closer look at some horses and, at one point, donning sunglasses and reaching up to his Aunt Meghan with his tongue sticking out.
Princess Charlotte impressed onlookers with her footballing skills, which she appeared to have been practising with her football-mad father.
Five-year-old Prince George, who is now on his school summer holidays joined in, and was seen climbing in a car boot as his siblings sat down for snacks.
Proceeds from the day will go to benefit African Parks, The English Schools Swimming Association, Fields in Trust, Henry van Straubenzee Memorial Fund, The Household Cavalry Operational Casualties Fund, The Invictus Games Foundation, Irish Guards Appeal, Map Action, Mountain Rescue England and Wales, The Queen's Commonwealth Trust, Rhino Conservation Botswana, The Royal Marsden, RFU Injured Players Foundation, The Passage and the Welsh Rugby Charitable Trust.