LONDON — When history’s great contests wind down, they leave questions that wars cannot answer and conflicts sometimes create.

That seems as true in the myriad centennial commemorations of the start of World War I this year as in the recollection this week of President Vladimir V. Putin, who invoked Russia’s indignities at the end of the Cold War to justify the annexation of Crimea.

It was a familiar reflex. As European history showed after the Treaty of Versailles in 1919, terms of peace that offer no dignity to the defeated sow the seeds of future conflict. In this century — witness the revived bloodletting in Iraq — wars that end on ill-defined terms merely store up the tinder of future conflagration.

So what is to be made of Britain’s drawdown in lockstep with the United States’ from a war in Afghanistan whose aims have shifted inconclusively as lives have been lost?...