BRUSSELS — Nearly two dozen European nations agreed Monday to extend the European Union's power into the military realm, approving a security pact that backers hope will boost defense cooperation.
The deal among most E.U. countries is an effort to give the E.U. the same clout on military matters that it has long held on trade and economics. Britain's decision to leave the E.U. has unsettled the 28-nation bloc, and President Trump has pushed Europe to spend more on its own defense.
Proponents say the deal is the biggest step forward in E.U. defense policy in decades and that it will open a path for robust military cooperation for years to come. Skeptics question whether the plan can succeed. They point to previous efforts at cooperation that fell by the wayside.
"What we can do together, we can do better than alone," said E.U. foreign policy chief Federica Mogherini, who masterminded the initiative that was signed Monday by defense and foreign ministers of 23 countries. "This is a historic achievement for European defense."
The initiative, known as Permanent Structured Cooperation, or PESCO, is an effort to forge a more assertive E.U. in defense matters. Britain's planned exit from the bloc opened the door to the initiative, since London had blocked previous proposals, saying they duplicated the NATO military alliance.
The agreement will create binding rules to boost defense spending and require European countries to cooperate with one another when they develop and buy new military equipment. Nations could also team up on military deployments and interventions.
Europeans have been criticized by U.S. and NATO leaders for taking a parochial approach to defense, favoring domestic manufacturers and allocating resources to deliver political dividends rather than concrete security benefits.
But in the Trump era, amid the deepest splits between Europe and the United States since World War II, many Europeans say they cannot rely on Washington for help.
"It was important for us that we Europeans stand up independently, especially after the election of the U.S. president," German Defense Minister Ursula von der Leyen said. "Nobody will solve our security problems for us. We have to do it ourselves."
The project would try to fight inefficient domestic spending by encouraging cross-border investments. Countries that join would also be expected to fulfill planning requests aimed at bolstering E.U. military capabilities as a whole. NATO also imposes similar requirements, leaving open questions about how the defense planning requests would be coordinated and what would happen if they contradicted each other. Most E.U. members belong to NATO and vice versa.
NATO leaders have endorsed the efforts, saying they support any plan to boost E.U. defense capabilities and spending.
"It's really about getting Europeans to be more capable on defense. And in order to be more capable on defense, given the size of our countries, we have to do it together," said Nathalie Tocci, director of the Institute of International Affairs in Rome and an adviser to Mogherini.
"It's a potential game-changer," laying the groundwork for an ambitious rethinking of the way Europe approaches defense issues, she said.
But defense experts said results may take time.
"On balance, the biggest changes, if any, will happen in the long run," said Tomas Valasek, director of the Carnegie Europe think tank. "The decision by itself isn't going to make countries behave differently tomorrow."
The plans are the result of a tug of war between France and Germany over the ambitions for the project. France wanted an exclusive war-fighting club of nations that could contribute troops and equipment toward expeditionary deployments far from European borders. Germany favored a more inclusive approach that its leaders said would provide a bigger jump-start to European defense more broadly.
In the end, the Germans prevailed, with only five E.U. nations sitting out the effort for now: Britain, Ireland, Denmark, Portugal and Malta. E.U. leaders still must give final approval to the initiative in December.