Prince William and wife Kate were pictured at the commemorative event in Liege, Belgium alongside other heads of state on Monday (4 August).
The event marks 100 years since the German invasion of Belgium, which led Britain to declare war on Germany.
Prince William and Kate - who were without baby son George - were greeted by Belgium's King King Philippe at L'Abbaye Saint-Laurent. From there, they made their way to Cointe Inter-allied Memorial for the service.
They will lay wreaths and take part in a minute's silence.
Prince William will also give a speech.
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