JERUSALEM — Israel’s defense minister, Ehud Barak, abruptly announced Monday that he will quit politics, potentially depriving Prime Minister Benjamin Netanyahu of a key ally who enabled his hard-line government to present a moderate face to the world.

Barring another comeback by the mercurial former general, Barak’s departure marked an end to a distinguished and tumultuous career that spanned half a century. It began on a communal farm, led to military greatness and business success and a mixed record in politics that was highlighted by failed peacemaking efforts during a brief term as prime minister.