David Cameron highlighted the ongoing defence and diplomatic co-operation between the UK and France as he joined world leaders marking the anniversary of the "extraordinary sacrifices" of D-Day.
The Prime Minister said the British and French had faced a common enemy in 1944 but the two nations remained important allies 70 years on, with personnel from both countries taking part in recent operations in Afghanistan, Libya and Mali.
The two countries had also acted together to increase diplomatic pressure over issues such as the Ukraine crisis and the continuing bloodshed in Syria, Mr Cameron said in an article for French newspaper Ouest-France.
The Prime Minister said: " As we gather on the beaches of Normandy to remember the extraordinary sacrifices made for peace, there has never been a more important time to underline our belief in collective defence.
"Through the searing experiences of moments like D-Day, we learnt how much more we could achieve by working together as allies than by fighting alone.
"The Nato alliance was born out of this commitment to increase our collective security and to ensure that the common cause we found through shared hardship would prevent conflict on this scale threatening our world again.
"Just as British and French soldiers fought for victory against a common enemy on the beaches of Normandy, today France and the UK stand shoulder to shoulder against the threats of the modern world. We remain united against international terrorism and extremism - and in recent times our armed forces have served together in Afghanistan, Libya, Mali and elsewhere around the world."
Mr Cameron said the "shared hardship" of the Second World War had "forged our unique relationship and created a shared determination to work together for a safer, more prosperous future for us all".
He added: "That future is why so many of our servicemen gave their lives - and protecting the peace they fought for is the greatest way we can honour those who fell."
Before the events commemorating the Normandy landings, Mr Cameron and French president Francois Hollande joined other leaders of the G7 nations in Brussels to warn Russia could face further sanctions and international isolation over its actions in Ukraine.
Mr Cameron said the UK and France had " worked together to ramp up diplomatic pressure in advancing our shared values, most recently in the push for humanitarian assistance in Syria and in our support for the Ukrainian government".