A national security expert details what’s being done, what can be done as the U.S. appears to rethink its decadeslong support.

Many European leaders believe they can no longer rely on the U.S. for the high level of defensive support they have counted on for decades now that President Donald Trump, a longtime critic of NATO and the European Union, has returned to office.

In this edited conversation, Richard D. Hooker Jr., a senior associate at the Belfer Center’s Project on Europe and the Transatlantic Relationship at Harvard Kennedy School, discusses what Europe has to do to get ready for that new reality.