Leaders of the NATO allies will hold a summit in December in London, Secretary General Jens Stoltenberg said on Wednesday. Britain is preparing to leave the European Union next month while asserting that it will retain a key role in the continent’s security. The last NATO summit, at the alliance’s Brussels headquarters in July, saw US President Donald Trump unsettle many European allies, including Germany, with demands that they spend more on their own defense. He also wanted the allies to reduce what he called their “dependence on Russia” for much of their energy needs. The meeting in London will be an opportunity “to address the security challenges we face now and in the future, and to ensure that NATO continues to adapt in order to keep its population of almost one billion people safe,” Stoltenberg said.