Nippon Steel Corp. won conditional U.S. approval for its $14.1 billion purchase of United States Steel Corp., capping a lengthy saga in a tie-up that will create one of the world’s largest steel companies.

His approval capped a tumultuous 18-month effort that survived union opposition and two national security reviews.

Nippon Steel Corp. won conditional US approval for its $14.1 billion purchase of United States Steel Corp., capping a lengthy saga in a tie-up that will create one of the world’s largest steel companies.