EU member states agreed to unlock what could amount to an unprecedented 800 billion euros ($867 billion) for defense spending amid mounting signs the U.S. is no longer a reliable ally, and the defense of Ukraine is falling squarely on their shoulders.

Replacing what the Americans do for the NATO alliance would take money, personnel, time and cooperation. None of those is easy.

European Union leaders committed to working together to bolster the continent’s defenses and to free up hundreds of billions of euros for security. The agreement emerged Thursday after U.S. President