PARIS — Besting all expectations, France’s leftist coalition pulled off an upset victory over the populist National Rally that quelled, for now, fears of a far-right takeover in a nation with a long history of keeping the far right out of government and at society’s fringes.

For more than half a century, France’s centrist and leftist parties maintained an effective firewall, known as the “cordon sanitaire,” or “Republican front,” against the far right. After the election’s first…