Yet when Britain votes June 23 on whether to leave the world's largest economic and political bloc, the pillars of Western security could be on the line.

A British decision to exit the 28-member union, experts say, could have disastrous repercussions for a U.S.-led post-Cold War order that relies increasingly on a united Europe to deter adversaries through economic sanctions, to pool intelligence about trans-national threats and to supply the diplomatic muscle to help contain conflicts.

While a British departure would not preclude the E.U. from those roles, the loss of a nation with the world's fifth-largest economy and defense budget would undoubtedly blunt Europe's punch. And it could presage a broader disintegration at a time when Europe's adversaries — from Russia to the Islamic State — already sense a level of vulnerability on the continent unlike any seen in decades.