Trident Juncture, an exercise involving many European nations, has been taking place in Norway.
Institutions which have helped keep the peace in Europe for over 70 years are under threat, a former chief of the defence staff has warned.
Independent crossbencher Lord Stirrup told the Lords that peace was fragile and could only be sustained by constant effort.
In a debate marking the centenary of the end of the First World War, he said: "A failure to unite, with all the messy compromises this entails, leaves us exposed and vulnerable to the dangers of an uncertain world."
Lord Stirrup joined peers from across the House in paying tribute to the sacrifices made by those who fought in the war, but warned it would be a "serious mistake" to bring the process to a close at the centenary of the Armistice.
He went on to say that the Allies' victory in 1918 had been turned into a "strategic defeat of the first order" diplomatic and political failures in the following years, leading to the "darker abyss" of the Second World War.
Lord Stirrup continued:
"We should be taking this opportunity to ask why.
"We should be discussing as a society the rise of selfish nationalism, the failure of international mechanisms, the unwillingness to confront challenges to international law and order."
Lord Stirrup said that post 1945, the victorious allies put in place processes and institutions like NATO to maintain peace through a unified purpose, rather than fragmentation and rancour. He said:
"Today the institutions that have served us so well for over 70 years are under threat.
"They should not be cast aside as so much obsolete bureaucracy."
Lord Stirrup said direct parallels could not be drawn between the two eras, but the sacrifice of those in the First World War should be used to "help us to do better" and ensure it was not in vain.
"That is why the centenary of the Armistice should not be an end but a new beginning for reflection, debate and learning."
For the government, Lord Ashton of Hyde said the war was fought on a scale difficult to comprehend but as well as marking the death and sacrifice that occurred, the nation should also recognise a hard won victory.
Meanwhile, Baroness Helic, a former adviser to William Hague when foreign secretary, warned that as memories faded she was "deeply worried that we have become forgetful of our history and reckless with our democracy", taking for granted the unbroken peace of the last 70 years and allowing "our moral radar to weaken".