According to The Hollywood Reporter:

Laurie will take on the role of Major de Coverley, the squadron executive officer on Pianosa air base. A noble, leonine presence, like some Civil War general, de Coverley dances to the beat of his own drum. Regarded with awe by the men. He spends his time pitching horseshoes, listening to jazz on his phonograph, mixing himself martinis and hiring apartments for the officers in every new city the Americans take.

Clooney, in his first series regular role since ER, will take on the role of Col. Cathcart, originally played by Martin Balsam. Clooney and his Smokehouse Pictures producing partner Grant Heslov (Good Night, and Good Luck, Argo, The Ides of March) will direct and executive produce. Ellen Kuras (Legion, Ozark) has also boarded the closed-ended miniseries as a producer and will direct two episodes. Luke Davies and David Michod will co-write and exec produce the series along with Anonymous Content’s Richard Brown and Steve Golin. A premiere date has yet to be determined. [My emphasis]