Its symbol, the poppy, has as much resonance now as it did in those years in the aftermath of the Great War as the nation grappled with collective trauma.
Around 1.75 million returned home with some kind of disability and on top of the appalling death toll among those who served were the widows, children, and bereaved parents.
There was a desire to honour the memory of the dead through giving help and support to the living.
Back then they had hoped to have fought a dreadful war to end all wars. It is a hope that was to be disappointed. The Second World War. Korea. Northern Ireland. The Falklands. Iraq. Afghanistan. And a host of small, nasty little conflicts and operations which are less well remembered.
While members of the public may have the impression that we are in modern times "at peace" the reality is that since 1945 there have only been a handful of years when there have been no British forces deaths as a result of hostile action, the last fatality being as recently as 2020.
That is just part of the story. Thanks to advances in modern medicine and battlefield evacuation, our wounded warriors are surviving terrible injuries which would once have inevitably been fatal. And as they recover and learn to live much altered lives they need continuing help and support – such as that offered by the RBL's innovative £27 million Battle Back Centre at Lilleshall, which officially opened in 2012.
Providing sports and adventure activities to promote self-confidence and improve motivation to aid recovery, it has been attended by over 8,000 people since it was established.
The coronavirus pandemic has posed challenges for charities but the fundraising is as important as ever. Sadly one consequence of Covid restrictions has been that as the Last Post sounded for many veterans they could not be honoured with a final farewell from RBL standard bearers.
The first Poppy Day was held in Britain on November 11, 1921. Today it remains the RBL's major fundraising effort, and with the easing of coronavirus restrictions collectors are back in local communities.
The foundation of the RBL a century ago came with a binding promise, that a grateful nation would remember its heroes. It is a promise which has been kept. Next month will see acts of remembrance at war memorials in cities, towns, and villages up and down the land. Those who served are also remembered at the legion's beautiful National Memorial Arboretum at Alrewas, a place to stroll, reflect, and give thanks.
This year's Poppy Appeal will be launched on October 28. Giving to the cause is not so much making a donation as repaying an enduring debt.