In a rare foray into international diplomacy, the second in line to the throne suggested the Ukraine conflict was not just a localised war but a threat to the whole of Europe.
'The events in Ukraine testify to the fact that instability continues to stalk our continent,' he said.
He spoke at the Cointe Inter-allied Memorial in Belgium, where he was representing the Queen to commemorate the start of the Great War.
While his father frequently puts his head above the parapet on sensitive international matters, William normally shies away from such controversial issues.
However, addressing the audience in Liege, among them dignitaries including King Philippe and Queen Mathilde of Belgium, French president Francois Hollande and German president Joachim Gauck, the young royal said: 'Edith Cavell was a British nurse who saved soldiers from each side.
'On the night before she faced a German firing squad she said, "I must have no hatred or bitterness towards anyone."
'It took another terrible war to learn the truth of her words, and even today we continue to learn that lesson.
'The events in Ukraine testify to the fact that instability continues to stalk our continent.' As the prince spoke, news was breaking that Russia's air force is to begin drills involving more than 100 aircraft from bases close to the Ukraine border.
Putin's annexation of the predominantly ethnic Russian region of Crimea - the first by a major power in Europe since 1945 - has already sparked international condemnation.