After six months of divorce negotiations, the European Union and the United Kingdom have reached a Brexit deal that allows them to move on to new discussions. On Dec. 8, the European Commission and the European Council announced that "sufficient progress" had been made on the three main issues of the first phase of Brexit talks: the United Kingdom's financial commitments to the union, the status of EU citizens living in the United Kingdom, and the border between Northern Ireland and the Republic of Ireland. This announcement opens the door to the second phase of negotiations, which will address two new topics: the transitional period after the United Kingdom leaves the European Union and the future trade deal between the two sides.

To achieve today's deal, the United Kingdom had to confront the issue of the Irish border, and it settled on promising that there will be no hard border imposed in Ireland. The border debate had been blocking negotiations for days, as the Irish government in Dublin demanded the border remain open while Northern Ireland's Democratic Unionist Party opposed any arrangements that would give the region special status in order to keep that border open. A document released Dec. 8 offers a compromise on the issue, stating that any future trade arrangements between the European Union and the United Kingdom should ensure that the border remains open. If this is not possible, the document says, London will "propose specific solutions to address the unique circumstances of the island of Ireland." One such solution could be to "maintain full alignment with those rules of the Internal Market and the Customs Union."

The language of the document indicates that the future of Northern Ireland will ultimately be shaped by the final trade agreement between the United Kingdom and the European Union, which still needs to be reached. Should London and Brussels fail to reach a comprehensive trade agreement, the question of the Irish border will reemerge. The British government insists that the United Kingdom will leave the single market, because it wants to regain full control of its immigration policy and free itself from the European Court of Justice. But it's unclear whether a situation where the United Kingdom leaves the single market but maintains "full alignment" with its rules would be enough to keep the Irish border open.

The complexity of the border situation explains why all the parties involved in Brexit talks reacted with caution to the agreement. European Council President Donald Tusk warned that if the breakup was hard, building a new relationship between the United Kingdom and the union would be even harder. Tusk also asked the British government to offer more clarity about the kind of relationship it wants to have with the bloc in the future. Irish Prime Minister Leo Varadkar, in turn, echoed a famous sentiment expressed by former British Prime Minister Winston Churchill, "this is not the end, but it is the end of the beginning." Meanwhile, other leaders took the opportunity to remind London of the domestic challenges ahead: After the deal was announced, Scotland's First Minister Nicola Sturgeon insisted that "any special arrangements for Northern Ireland must be available to other U.K. nations."

The next step of the Brexit process will take place Dec. 14-15, when the leaders of the European Union will hold their last summit of the year. During this meeting, the 27 governments are likely to authorize the start of the next phase of Brexit talks, focused on the transitional period and the trade agreement. Negotiations on both issues are likely to consume most of 2018, especially considering the differences between trade agreements for goods versus for services. In general, trade agreements are easier to negotiate for goods than they are for services. But considering the size of the United Kingdom's financial sector, London will likely push to include services as well, which could require particularly complex discussions throughout next year.