Japanese steel giant Nippon Steel $5401.T has been pursuing the acquisition of U.S. Steel $X for over a year and a half. The potential $15 billion cross-border transaction has encountered resistance from American labor unions and has undergone two national security reviews. This week, the long-simmering deal took a potentially decisive turn as former U.S. President Donald Trump is set to lead a rally in Pittsburgh, Pennsylvania, the city where U.S. Steel is headquartered.