BRUSSELS — European Union countries on Monday launched a new era in defense cooperation with a program of joint military investment and project development aimed at helping the EU confront its security challenges.

Twenty-three of the EU’s 28 member nations signed up for the process, known as permanent structured cooperation, or PESCO. Britain, which is leaving the EU in 2019, and Denmark with a defense opt-out were among those not taking part.

EU foreign policy chief Federica Mogherini described it as a ‘‘historic moment in European defense,’’ and added that ‘‘23 member states engaging both on capabilities and on operational steps is something big.’’ Those who didn’t sign up can join later.

Mogherini said countries have already submitted more than 50 joint projects in the fields of defense capabilities and military operations.