John Sullivan, former deputy secretary of state and U.S. ambassador to the Russian Federation, rejoined Mayer Brown as a partner and co-leader of the firm’s national security practice, which he co-founded with fellow co-leader Raj De in 2015. Sullivan will split his time between the firm’s New York and Washington, D.C., offices.

Coming from the public sector, Sullivan said that he likely will be splitting his time between adding value to the firm’s existing clients while also building up his own book of business.