So far, so surprisingly good. But staying united on Brexit is about to get harder for the EU's remaining members.

From the moment last June when the British voted to ditch the European Union, senior officials in Brussels have labored fastidiously to keep the rest of the bloc together.

Given that disagreement, debate and discord are encoded in the EU's DNA — a genetic mutation supporters of the European project call democracy — they have had remarkable success.

At a summit in Brussels on Saturday, leaders of the 27 countries staying in the EU swiftly approved their guidelines for the Brexit negotiations. It took one minute or four minutes, depending on whose account you believe, but the key message from EU officials was that approval was quick and unanimous.

"EU27 firm and fair political mandate for the #Brexit talks is ready," European Council President Donald Tusk tweeted.

Perhaps no one has been more astonished, or disappointed, at the EU's ability to hold together than U.K. Prime Minister Theresa May, who warned this week that the 27 are lining up "to oppose us."

And no one knows better than the 27 leaders gathered in Brussels today how hard, if not impossible, it will be to keep up the unified front as the two sides plunge into the detailed negotiations.

European Commission President Jean-Claude Juncker predicted that the Council would never be as unified as it was Saturday. "It was the first and the last time that we were able to conclude in four minutes, so don't expect us to keep the same speed," Juncker said at a news conference immediately after the conclusion of the summit. "It will never happen again."

Developing the guidelines was the relatively easy part, largely because they emphasize points all EU countries agree on: the need to negotiate withdrawal terms before agreement on future relations, and prioritizing citizens' rights, the financial settlement, and border concerns — particularly for Ireland.

Now the hard part begins, and the risk of cracks is already clear in how countries have begun jockeying for the two EU agencies, the European Medicines Agency and the European Banking Authority, that will move out of London.

"Mike Tyson said 'the battle plan doesn't survive the first punch in the nose,'" a senior EU diplomat said. Actually, it was Joe Louis who said "everybody has a plan until they get hit." But point taken.

The punches are coming.

The risk of disagreement among the EU 27 on detailed aspects of the Brexit negotiations is high, largely because the interests of individual countries in the Brexit talks diverge as much as they do on any other issue.

Different priorities

When it comes to citizens' rights, for instance, countries such as Spain and Malta have a primary interest in the fate of British retirees who live in those countries, and the related costs for health care and other services. Countries such as Poland and Lithuania are far more concerned about their own citizens now living and working in the U.K., who send crucial remittances home.

One EU diplomat suggested that it would be better to let the negotiators get on with their work and limit the number of meetings among diplomats and higher-level officials.

Some EU countries, such as Belgium and the Netherlands, are nearly as eager as the U.K. to move to discussions on a future free trade agreement. France and Germany, the biggest economies of the 27, can afford to demand that Britain feel some pain from leaving the EU's single market.

“The most divisive issues are not on the table yet,” Lithuanian Foreign Minister Linas Linkevičius said in an interview with POLITICO.

There were also signs that EU countries were beginning to use Brussels' desire for unity on Brexit as leverage in negotiating concessions from the European institutions on other matters.

“Countries are already starting to link issues that are off the table with the Brexit talks," an EU diplomat said. "They start sending the message that if you want to keep unity on Brexit then they have to get this or this other thing on other dossiers. I expect that this will increasingly be the case."

As if to underscore the point, Italian Prime Minister Paolo Gentiloni told reporters at the summit that maintaining unity on Brexit would be easier going forward if the EU adjusted its policies on fiscal austerity and migration.

To build the unity on display at Saturday's summit, senior EU officials began working immediately after the Brexit referendum last June. They held an informal EU 27 summit in Bratislava, where they laid out a roadmap for the Brexit process ahead. They also used the occasion of celebrating the 60th anniversary of the Treaty of Rome, one of the EU's foundational documents, to rally countries and express renewed common cause.

Tusk also acknowledged how unusual it was for the EU to have everyone on the same page, and he expressed confidence that the togetherness would last.

At the Commission, Juncker has also led a bold push to focus the 27 on the EU's common future by publishing a well-received White Paper laying out various options for the bloc.

"The Commission’s approach has been particularly intelligent and sophisticated," an EU diplomat said. "You should see the Juncker White Paper as a very important flip side process of the Brexit negotiations and the need to ensure that member states buy into this project of continuing EU unity."

But the ability to continue such hand-holding of the 27 leaders will be vastly reduced once the formal negotiations begin in June. The talks are expected to quickly become intensive, with negotiators potentially working six days a week in a race against the clock to clinch an accord within the two-year deadline set out in the EU treaties.

Devil in the directives

Already, the discussions among EU diplomats over more detailed "negotiating directives" being developed by the European Commission are more complicated than the talks on the broader guidelines.

One EU diplomat suggested that it would be better to let the negotiators get on with their work and limit the number of meetings among diplomats and higher-level officials.

"The original text was so well done, so well oriented, that from Poland to Sweden there haven’t been substantial" differences, the diplomat said. "Sometimes even if you have nothing to object [to], you start discussing for the sake of discussing — so let’s better not even start getting into a meeting-frenzy."

Before and after Saturday's summit, Tusk emphasized the importance of holding ranks, and seemed to send May a subtle warning not to try to split the 27, by saying that would be bad for Britain.

"If someone expected some divisions among the 27 would help the U.K. to achieve something better for them, it's a pure illusion," Tusk said at the closing news conference. "The only possible way to achieve a final agreement between the 27 and the U.K. is unity of the 27. I have no doubts this is the first and most important political condition."

Jacopo Barigazzi, Jakob Hanke, Maïa de la Baume, Quentin Airés, Carmen Paun and Joanna Plucinska contributed reporting.