Only certainty of Brexit is it brings more uncertainty

European Commission President Jean-Claude Juncker called it "the best deal possible". British Prime Minister Theresa May said it was the "only possible deal".

But, significant as it is, the Brexit deal the other 27 EU leaders endorsed on Sunday in Brussels is no guarantee that the United Kingdom's withdrawal from the union will proceed smoothly. Instead, tougher bargaining awaits May both at home and with the European Union going forward.

What EU leaders approved on Sunday was a treaty setting the official terms for Brexit in March and an outline of future trade ties. Being an outline means numerous specifics at technical levels need to be sorted out. That is precisely where the toughest negotiations will be. Only they do not take place until after March, when the UK has officially departed.