The onetime dynamo is fighting to revive a takeover by Nippon Steel. Other tie-ups could also face obstacles, and going it alone could force cutbacks.

Even if losing domestic sources of primary steel does not seem like a national security threat today, it might tomorrow.

U.S. Steel and Japan-based Nippon Steel sued the Biden administration on Monday over a decision made last week to block a merger between the two companies. A proposed $14 trillion acquisition of the second-largest domestic steel producer by Nippon would threaten U.