EU needs to promote political stability to avoid a WWIII: George Mitchell

European Union leaders, following Britain's exit vote, need to promote the political stability that's helped keep the Continent in relative peace for 65 years, ex-Democratic Sen. George Mitchell told CNBC on Thursday.

The former Senate majority leader and diplomat told "Squawk Box" that any economic fallout from a Brexit is going to be down the road, but avoiding the kinds of conflicts that led to World Wars I and II is the most pressing concern for Europe.

"In World Wars I and II, 68 million people died, in a world in which the population was a third or a little more than what it is today," Mitchell said. "The prevention of that, I think, is a valuable attainment." The current population of the world is around 7.4 billion people.

Founded in 1951, the EU has 28 members, including Britain. Several subsequent expansions of the group have led to the participation of most of Europe.

"The EU is at heart a peace effort, part of the post-war effort ... to create a series of political, military ... and other entanglements that would make less likely a resumption of conflict in Europe," Mitchell said.

"It's been very successful in that regard," he argued, but acknowledged, "it's awkward, clumsy — 28 countries trying to agree on something, [and] everybody with a veto power."

A week ago Thursday, Britons went the polls and shockingly voted to leave the bloc. World financial markets were sent into a tailspin, racking up more than $3 trillion in paper losses in two days.

But some of those immediate concerns abated on Tuesday and Wednesday, as stocks from Wall Street to London rallied, making up the bulk of those market declines.

Mitchell is no stranger to complex geopolitical situations, having served as U.S. special envoy to Northern Ireland from 1995 to 2001 and U.S. special envoy for Middle East peace from 2009 to 2011.

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