EU bids sad farewell to United Kingdom

"It is a sad day," European Commission President Jean-Claude Juncker said during the two-hour special meeting of the European Council. "It is no reason to celebrate when a country leaves the EU. It is a tragedy."

The Brexit deal to end the UK's 46-year membership in the EU was agreed to unanimously by the EU's 27 member states. The end of 21 months of tough negotiation left no one in a party mood. "This is not about who won or lost," European Council President Donald Tusk said ahead of the meeting.

In a press conference afterwards, Chancellor Angela Merkel spoke of mixed feelings. "It brings sadness, but also a bit of relief," she said, calling the deal an unprecedented "diplomatic work of art" that managed to accommodate everyone's interests. It was "no time for champagne," said European Commission President Jean-Claude Juncker, because a separation "is never easy."

Read more: No-deal Brexit fears on minds of German manufacturers

May set for battle in Westminster

The UK side did not reflect the mournful attitudes. The deal "sets us on course for a future of opportunity and prosperity," Prime Minister Theresa May said, promising advantageous trade deals around the world, and more money to spend on domestic issues like health care. "We will take back control."

The UK Parliament is likely to vote on the deal before Christmas, and the vote will not be easy for May to win. "This is the best deal," she insisted, adding she "looks forward" to campaigning for it. But with bitter criticism back at home and no chance of renegotiation, she declined to say if she would resign should the deal not pass Parliament. "This is not about me."

Remaining would be better

"The best for Ireland and Great Britain would be to remain in the EU, the single market and the customs union," said Ireland's Prime Minister Leo Varadkar. His is the only EU country to share a land border with the UK, for which a "backstop" has been agreed to, final deal or not. The deal would keep the border open between the Republic of Ireland, an EU member, and Northern Ireland, which, as part of the UK, is set to leave the EU on March 29, 2019.

That backstop has been the repeated sticking point in negotiations. Brexit hard-liners in May's conservative government have rejected any exception for the intra-Irish border, saying it would keep the entire United Kingdom stuck under EU customs rules for an indefinite period of time. The EU has refused to reinstate a hard border, given the risk that would pose to the Good Friday Agreement, which helped end decades of sectarian violence in Northern Ireland.

Negotiations through 2020

The deal the 27 EU member states have now approved would see to an orderly divorce, whereas a hard or no-deal Brexit could have chaotic consequences for trade, travel and business on both sides of the English Channel. A trade deal between the EU and UK may not happen until the end of 2020, skeptical EU diplomats said. Those negotiations would begin once the UK officially leaves the EU in March. The transition period to follow could be extended up to two years.

"Today's agreement is just another necessary step along an unprecedented path to build trust," said Michel Barnier, the EU's Brexit negotiator.

Fishing policy and the status of Gibraltar are additional points of contention, especially with France and Spain, respectively. Spain wants a final say over post-Brexit relations with the rocky outcrop adjoined to its southern coast that is controlled by Britain, and voted overwhelmingly to remain in the EU.

Read more: Gibraltar dispute shows the absurdity of Brexit

UK Parliament is the real hurdle

May faces the real possibility of the deal now being struck down by her own parliament. She currently lacks the votes, and her Conservative Party's junior coalition partner, the Democratic Unionist Party from Northern Ireland, has expressed its opposition to the deal and is threatening to bring the government down for it.

"This is the best deal for the UK, the best deal for Europe, this is the only deal possible," Jean-Claude Juncker said. Officially, EU diplomats have said there will be no further negotiations, although some have suggested a degree of additional flexibility.

"Of course the EU will negotiate until the last minute because we don't want to be responsible for a hard Brexit without any deal," one EU official said. "It's the British who say when it's done."