During a 24-hour swing through Copenhagen last month, Eric Slesinger met with engineers making maritime drones, developers of war-planning software and an adviser to NATO. He had recently visited London for a dinner with a senior British intelligence official and would soon head to the Arctic to learn about the technologies that could handle extreme climates.
The packed schedule would seem more common for Mr. Slesinger in his former job as an officer at the Central Intelligence Agency. But now the 35-year-old was in high demand as he parlayed his spy agency credentials into a career as a venture capitalist focused on the suddenly relevant area of defense and national security technology in Europe.
“This is all happening at warp speed,” said Mr. Slesinger, who has backed eight defense start-ups and has negotiated with several more.
As President Trump throws the future of the trans-Atlantic relationship into question, governments across Europe have outlined plans to potentially spend hundreds of billions of euros on weapons, missile-defense programs, satellite systems and other technologies to rebuild their armies. Technologists, entrepreneurs and investors are racing to take advantage of the spending boom by creating new defense start-ups.
Few paid attention several years ago when Mr. Slesinger moved to Madrid with the idea that Europe would need to drastically increase defense spending because U.S. military protection could not be taken for granted. Now his predictions look prescient. After Mr. Trump’s inauguration, which followed his defeat of Vice President Kamala Harris in the November election, members of his administration called Europe “pathetic” and military mooches of the United States.