William and Kate joined the Queen and Duke of Edinburgh at a wreath-laying service held at a war memorial on the monarch's Sandringham estate.
The Cambridges were not scheduled to attend the ceremony which marked the 100th anniversary of the final withdrawal of allied troops from the Gallipoli peninsula in the First World War.
Kate, who celebrated her 34th birthday on Sunday, was joined at church by her parents Michael and Carole Middleton and siblings Pippa and James.
Then, with solemnity and quiet dignity, the Queen and Duke each placed a wreath at the war memorial dedicated to local men who fought in the Great War and paid the ultimate sacrifice.
At dawn on April 25, 1915, waves of Allied troops launched an amphibious attack on the strategically important 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 was flawed and the campaign, which faced a heroic defence by the Turks, led to stalemate and withdrawal eight months later.
Some 44,000 Allied troops died, including 8700 from Australia and 2700 New Zealanders. Almost 87,000 Turks lost their lives.
The last Allied troops were withdrawn on January 9, 1916.
