German Defense Minister Ursula von der Leyen attends a news conference in Berlin in July. She says that Britain’s decision to leave the European Union offers an opportunity for greater military cooperation in the bloc. (Markus Schreiber/AP)

European leaders gathered Friday to discuss the torrent of challenges facing their continent after Britain’s decision to leave the European Union, offering broad new defense efforts amid growing concerns that they are too dependent on the United States for security.

Deeply divided about how to keep the bloc from further spiraling apart, leaders found little unity on a specific vision for a Europe without Britain. But there was more agreement that Europe should increase military coordination, after a tough push from both President Obama and Republican presidential nominee Donald Trump over their lagging defense spending.

The proposals, including a Franco-German idea to create a centralized European military headquarters in Brussels, were part of a wide-ranging conclave held in Bratislava, Slovakia, on how to combat a rising tide of public skepticism about the European Union’s value. Security has become a central focus, but military spending in most E.U. nations also falls below the levels desired by NATO: Only four NATO countries apart from the United States meet the bar, and the United States spends more than twice as much as all the other nations combined. 

Although E.U. nations have mustered other joint military efforts in recent years, they have often been criticized for being slow to move during crises, reflecting the difficulty of mobilizing members of a 28-nation bloc on an ad hoc basis.

Stronger coordination among European countries could bolster militaries that have relied on American firepower as a crutch, reducing duplication among countries in the name of creating a continent-wide force to defend the bloc of 500 million citizens. 

“Let everyone know that if the United States makes a choice to pull back, Europe must be able to defend itself,” French President François Hollande said Friday as he entered the talks.

The endeavors could offer a retort to both Democrats and Republicans, who have questioned Europe’s reliance on the United States for defense. But critics say resources could be better spent in partnership with NATO, the existing military alliance that includes the United States and most E.U. countries. A Europe that is more independent militarily would also be more capable of pursuing a foreign policy path more distinct from the United States, potentially widening cracks in Western unity, although few experts believe this is likely.

The attempt to improve security was a rare spot of unity for a summit of 27 leaders — British Prime Minister Theresa May was not invited — with seemingly 27 different plans for Europe. The discussion comes after the migration crisis swept millions of asylum seekers into Europe last year, fueling worries from Paris to Prague that the borderless E.U. was a burden, not a boon.

“We are in such a critical situation in Europe after the referendum in Britain,” German Chancellor Angela Merkel told reporters after the day-long meeting that was held in part on a pleasure boat that cruised down the Danube. She said leaders had agreed to put together a reform plan by March of next year.

The leaders made little secret of their disagreements, from the virtues of economic austerity to Brussels’ role in handling threats from Russia and the Islamic State. The discord left little room for an older, loftier view of the E.U., rooted in its post-World War II history as a project to bring peace to Europe.

“I am not going to follow a script to show that we are all united,” Italian Prime Minister Matteo Renzi said after the meeting broke up. The summit accomplished “too little,” he added on Twitter. “Without changing policies on the economy and immigration, Europe risks a lot.”

Ahead of Friday’s meeting, France and Germany unveiled a proposal for increased cooperation that would create a permanent headquarters to run E.U. military operations. Those military efforts currently include an anti-people-smuggling operation in the Mediterranean and an anti-pirate campaign off the coast of Somalia.

European nations would also team up to bolster capabilities in areas where they lag, such as air-to-air refueling. In the past, they have been forced to turn to the U.S. military for assistance. E.U. nations would also share aerial reconnaissance information and work to make combined European military battalions more ready to deploy into combat at short notice.

Other E.U. leaders have proposed jointly owned drones and other commonly held equipment that could supplement European deployments.

The discussions about bolstering defense cooperation are supported by an unusual coalition of leaders. Central European leaders who have been some of the most strident opponents of taking in migrants have embraced the idea of a full-fledged E.U. military force, warning that Britain’s exit from the E.U. will significantly sap the continent’s defense capabilities.

Britain, which has Europe’s most powerful military, long blocked any discussion of centralizing European military might, in part because the E.U. was so unpopular domestically. The dissolution of the Soviet Union also left many E.U. nations feeling that they faced no obvious military threat.

But Russia’s annexation of Crimea renewed fears along Europe’s eastern flank. And the migration crisis forced a recognition that Europe’s borders were being defended by some of its weakest countries.

The efforts to pool European defense would touch one of the core aspects of national sovereignty: the military, years after many European countries agreed to share a single currency and rely on one another for border controls.

In those fields, many European citizens feel stung after the euro crisis highlighted sharp differences in how best to spur growth and the migration crisis ushered a stream of refugees into Europe amid disagreements about how and whether to house them. 

E.U. leaders warn that if the bloc fails to ensure security for its citizens, it could rapidly fall apart. The Syrian conflict may once have seemed far away to European citizens, but not after last year’s flood of migrants and a cascade of Islamic State-inspired terrorist attacks.

“The borders between internal and external security have simply evaporated,” said Jo Coelbart, a retired Belgian general who works on European defense policy at the Brussels-based Egmont Foundation.

That challenge has spurred the new defense proposals.

“It was a sad moment for Europe when the British people decided to leave, and so it requested an honest diagnosis,” said European Council President Donald Tusk, summing up conversations that extended beyond security to issues of prosperity, migration and a host of other challenges. “People are concerned about what they see as a loss of control.”

Despite the range of views, defense cooperation may be among the least-divisive projects for Europe’s future, particularly with American commitments to European security the most questionable since the end of World War II. Trump has said that he would not automatically come to the defense of other NATO alliance members but would first review how much those nations have done for their own defense. Democratic candidate Hillary Clinton, like Obama, has encouraged European nations to bolster defense spending but has said she will hold firm to U.S. defense commitments.

“With Trump jumping up and down saying he’ll tear up the NATO treaty if the Europeans don’t shape up, there’s a desire to show progress,” said Nick Witney, a fellow at the European Council on Foreign Relations who ran the European Defense Agency, an E.U. institution whose efforts face strict limits because of the European reluctance to hand over security powers to Brussels.

“There are plenty of things that could be done to get more bang for the euro by avoiding the endless national duplication,” he said.