Anzac Day - April 25 - marks the anniversary of the start of the First World War Gallipoli landings, and is a national day of remembrance for Australia and New Zealand
Thousands of Anzac troops - Australian and New Zealand Army Corps - died in the ill-fated 1915 campaign.
Waves of allied forces launched an amphibious attack on the strategically important Turkish peninsula, which was key to controlling the Dardanelles straits, the crucial route to the Black Sea and Russia.
But the plan backed by Winston Churchill, then first lord of the admiralty, was flawed and the campaign, which faced a heroic defence by the Turks, led to stalemate and withdrawal eight months later.
Its legacy is the celebration of the "Anzac spirit" - courage, endurance, initiative, discipline and mateship - shown by the Antipodean troops.
William highlighted these qualities in a message to mark Anzac Day sent to the New Zealand and Australian High Commissions in London, along with some Anzac biscuits, on Friday morning.
On Sunday, the Princess Royal, and her husband Vice Admiral Sir Tim Laurence, were attending two commemorative services in London on behalf of the royal family.
The duke said in his message: "This Anzac Day, Catherine and I join Australians and New Zealanders across the world to remember and honour the servicemen and women of the Australian and New Zealand Army Corps.
"Today we stand together to reflect not only on their sacrifices, but also their courage, sense of duty, and their famously indomitable spirit.
"Though many will still be unable to come together in person this year, we are heartened in the knowledge that Australians and New Zealanders will continue to commemorate those who have given so much for our freedoms.
"The Anzac qualities of endurance, courage, ingenuity, good humour and mateship are admired as fiercely as ever before.
"Lest we forget."
Anne and her husband were among a reduced number attending the Anzac Day Dawn Service at Wellington Arch, where the princess laid a wreath at both the Australian and New Zealand War Memorials, and signed a book of remembrance.
Later, the couple will attend the Anzac Day Service of Commemoration and Thanksgiving at Westminster Abbey.
Anzac Day has been commemorated in London since the first anniversary of the Anzac landings when King George V attended a service at Westminster Abbey.
Since then, the services have become an important moment for thousands of expatriates and visiting New Zealanders and Australians.