You may have missed it amid headlines of the U.S. trade war with China, but a low-intensity trade conflict with Asia’s other emerging giant is also smoldering. It began a year ago when the U.S. stripped India of special low-tariff status over claims it discriminated against American companies. India responded by implementing tariffs it had previously threatened over U.S. steel duties.

President Trump travels to India next week where he may reach some sort of truce with Prime Minister Narendra Modi. But the underlying conflict...