Because Scotland and Northern Ireland want to remain part of the E.U., there is the quite real possibility that Scotland and even Northern Ireland might now choose to go their own way on membership within the E.U. and the 'United Kingdom' would suddenly effectively be only England and Wales.
The E.U. is the latest of a series of multinational organizations set up after World War II to ensure that there would never again be a pan-European war and to create the conditions for a new European prosperity after the destruction wrought by the war against the Nazis. The E.U. has admirably succeeded at both.
President-elect Donald Trump has a host of national security challenges to deal with as he assumes office, from the resurgence of the Taliban in Afghanistan to the grinding Syrian civil war to the flexing of Russian muscles under President Vladimir Putin to how to deal with ISIS as the terrorist army retreats in Iraq.