Indian Prime Minister Narendra Modi promised to unite the country on Thursday after a big election win, with his party on course to increase its majority on a mandate of business-friendly policies and a tough stand on national security. Official data from the Election Commission showed Modi’s Hindu-nationalist Bharatiya Janata Party (BJP) ahead in 300 of the 542 seats being contested, up from the 282 it won in 2014 and more than the 272 seats needed for a majority in the lower house of parliament. That would give it the first back-to-back majority for a single party since 1984. “Together...