European Council President Donald Tusk declared himself the "guardian of European unity" at EU leaders' fall summit on Thursday evening.

Judging by a renewed appetite for policy fights among the leaders, he may have his work cut out for him.

Buoyed by a sense that the bloc is back in business after putting up a united front in the face of Brexit, leaders weighed in on big issues such as migration, the digital economy and energy.

But tackling such tough topics also brought divisions to the surface, threatening the very unity they have shown over the past year.

The leaders engaged in a fierce debate over a proposal on taxing digital behemoths like Google, Apple and Amazon, which delayed for an hour Tusk’s joint news conference with Commission President Jean-Claude Juncker.

The proposal for a new tax system, championed by France, is fiercely opposed by countries such as Ireland and Luxembourg, which have benefited by offering low tax schemes to digital giants that the Commission and other countries say are unfair or illegal. Officials said the leaders’ discussion included an animated clash between Irish Prime Minister Leo Varadkar and French President Emmanuel Macron.

There were also some disagreements on a broader digital policy package, with some leaders expressing great impatience at the slow pace of implementation.

And there was griping over energy policy and Russia’s Nord Stream 2 gas pipeline, which Poland and Denmark fiercely oppose but the Commission’s legal service has said could not be blocked under EU law. A senior EU official said the Commission would next month come forward with a proposal seeking to revise energy laws to bring the pipeline under EU jurisdiction.

Juncker, however, expected further discord on the topic. "I don’t feel that we will have unanimity on this in the coming months," he said at the news conference. "But the Commission continues to feel it will be useful for it to be granted a mandate for discussions with Russia.”

Also on display at the summit were divisions on migration policy and on the bloc’s frayed relations with Turkey. On migration, the Commission expressed frustration that national leaders have not yet come forward with billions in additional funding that officials said was needed to close off the Central Mediterranean route used by illegal migrants from Africa.

Since the U.K.’s vote to leave in June 2016, the EU has worked aggressively — and successfully — to maintain unity among the remaining 27 countries.

The leaders also postponed a discussion and decision on a replacement for the “Dublin” system on how to handle the processing and relocation of asylum seekers that has severely split the bloc, and that leaders seem unlikely to resolve anytime soon.

Catalonia concern

On the summit sidelines, there were renewed tensions between Spain and Belgium over Catalonia. Belgian Prime Minister Charles Michel, who must contend with Flemish separatism in his own country, had expressed concern over the force used by Spanish police to suppress the illegal independence referendum in Catalonia earlier this month. Otherwise, the EU had broadly supported Madrid and Spanish President Mariano Rajoy.

As a result of Michel’s comments, the Spanish foreign mMinistry said it would not back Catherine De Bolle, the head of Belgium’s federal police, in a bid to become president of Europol.

Since the U.K.’s vote to leave in June 2016, the EU has worked aggressively — and successfully — to maintain unity among the remaining 27 countries. And leaders have demonstrated unwavering discipline in their negotiations with Britain, insisting without any hint of disagreement that the three core divorce issues be addressed before any talk of a transition or future trade relationship.

But there was an unmistakable undercurrent of tension leading up to this week's summit, as leaders digested a series of speeches full of big ideas about the future of Europe, particularly one by Macron at the Sorbonne, as well as Juncker’s State of the Union speech in Strasbourg. Many countries feared that the new French president was too eager to endorse use of the so-called multispeed Europe strategy, under which some countries could pursue cooperation on contentious issues without securing the traditional unanimity in the European Council.

“I would like to state very clearly that as long as I am here, I will be the guardian of European unity" — Donald Tusk

There were also concerns among some leaders that the election of Macron, combined with the reelection of German Chancellor Angela Merkel, would lead to a renewed Franco-German power axis that would be used to press the agenda of bigger countries, and potentially bully smaller ones.

Arriving at the summit, Swedish Prime Minister Stefan Löfven said that he preferred a Franco-German alliance and partnership to any animosity between the bloc's biggest powers, but he also stressed the need for other countries to be heard, and for the bloc as a whole to remain unified. "We all have a voice, especially here in the Council," Löfven said.

While the EU treaties not only envision but authorize such an approach, a number of leaders, particularly from Eastern Europe, fear that it would lead to unequal tiers of EU membership.

Tusk, in putting forward a “leaders’ agenda” which incorporated many of Macron’s proposals, stressed that unity remained a priority and that Macron had given him assurance that it would be the default option.

“The objective of this plan is to move forward in Europe on key issues, like security or migration, while keeping our unity,” Tusk said at the news conference on Thursday, following the close of the first series of policy discussions.

“And I would like to state very clearly that as long as I am here, I will be the guardian of European unity,” he said. “It is not only my formal role as the president of the European Council, but — above all — it is my true belief. Because unity is, in fact, our most important strength.”

Consensus and controversy

Senior EU officials said the vigorous policy debates reflected the bloc's renewed confidence after steering through a series of existential threats: the eurozone crisis, the migration crisis, and the initial stages of Brexit.

"It was very consensual today,” said a senior EU diplomat from a Central European country. “Everybody was contributing in a positive manner."

On migration, the tone was upbeat as Tusk said that "we have a real chance of closing the Central Mediterranean route” and praised Italy's work with Libya, which has resulted in a 25 percent drop in irregular migrant arrivals in the first 10 months of 2017 compared to last year, according to data from the Italian interior ministry.

And yet reform of the "Dublin" system meant to manage asylum claims continues to be postponed.

In the Council conclusions of December 2016, Malta was charged with "achieving consensus" on Dublin during its six-month EU presidency. Almost a year later, Thursday's summit conclusions said the Council "will seek to reach a consensus during the first half of 2018."

The issue is still so divisive that Michel, the Belgian prime minister, threatened during a news conference to force a decision using "qualified-majority voting," the mechanism available to the Council on certain issues when unanimity cannot be achieved. It was the use of qualified majority voting in September 2015 to approve a mandatory relocation scheme for 120,000 refugees that triggered an angry reaction from Hungary, Slovakia and later also Poland, and splintered the EU.

Ireland, Luxembourg and Malta are just some of the countries that have been outspoken against an EU-only proposal on digital taxation.

Tensions are already rising. "A rush to the use of qualified majority has to be mindful of consequences” a diplomat from Central Europe said.

On the digital taxation issue, Macron in the end secured a last-minute change to the summit conclusions, putting pressure on countries to accelerate discussions on a new European digital tax proposal. The new conclusions, with Macron’s pressure, call for a Commission proposal by early 2018.

But Macron faced significant pushback from a number of leaders, especially Varadkar. Ireland, Luxembourg and Malta are just some of the countries that have been outspoken against an EU-only proposal on digital taxation.

Varadkar told leaders he wanted assurance there was still a mention in the conclusions of working with the Organization for Economic Cooperation and Development to ensure a global approach on digital tax. While there is still mention of the OECD in the conclusions and the need to keep their position in mind, the final tone insists there needs to be an EU-wide effort regardless.

"It was probably the most intense point, but in a positive way," Michel said at a briefing later. "It was the occasion to note, with some frankness, without any unsaid things, which is a good thing for me, that some countries are still reluctant — we can feel it — to move forward on a common taxation strategy to tax internet platforms."

Carmen Paun and Judith Mischke contributed reporting.