Cameron said the European Union had helped maintain peace on the continent and Britain should vote to remain a part of the 28-member economic bloc in the June 23 referendum.
"The European Union has helped reconcile countries which were at each others' throats for decades. Britain has a fundamental national interest in maintaining common purpose in Europe to avoid future conflict between European countries. And that requires British leadership, and for Britain to remain a member," he said in a speech at the British Museum here.
He ranked 2016 alongside other major events in European history, including the Spanish Armada in 1588, the battles of Blenheim and Waterloo in 1704 and 1815 respectively, the two World Wars, as well as the fall of the Berlin Wall.
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Today's major speech came as the referendum campaign heats up with just over six weeks to go.
The opposing Vote Leave campaign immediately attacked back, saying: "During the renegotiation the PM said he 'ruled nothing out'. Now he thinks leaving the EU would lead to war. What changed?"
Polls showed the two campaigns were neck-and-neck.
HeaccusedCameron of "corroding" trust in British politics by saying he will cut netimmigration to tens of thousands and failing.
"It isdeeply corrosive of popular trust in politics that every year people in power say they can cut immigration to tens of thousands and then fail. We have absolutely no power to control the number of people coming. I am in favour of immigration but also in favour of control," he said.