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.

Before that, the British prime minister faces the challenging task of pushing her deal through a less-than-supportive parliament.

May argued she has maneuvered the best deal that was possible, one that she says best serves the UK's interests and maintains a mutually beneficial partnership with the EU. But not everyone at home buys that claim. Given the dissent of her many vocal critics, she has a difficult fight ahead of Dec 11, when the House of Commons is expected to vote on the deal.

If she fails that test, and the probability cannot be excluded at this point, and if the EU insists on its current No-Plan-B position, things will turn very ugly. As one senior EU official was quoted as saying, the only Plan B was preparing a possible no-deal scenario where both the UK and the EU will suffer, not just economically.

To most observers, the UK has shot itself in the leg, and is busy arguing over the best way to treat the wound while it continues to bleed out. Unfortunately, that has consequences not only for itself, and the only certainty regarding Brexit and the UK-EU relationship in the next few months is there will be more uncertainty.

Along with that is the inevitability of broad reflections on the UK's identity as a European country, EU member, and its relationship with the rest of Europe, and possibly the need to rebuild it.