Last Monday, India’s central bank banned local companies from doing business with the United Nations’ Asian Clearing Union—a clear slap at Tehran, which uses the opaque organization to evade U.N. and U.S. sanctions. Delhi didn’t cite that as the reason for its actions; its statement noted only “difficulties” in payments and receipts. But Iran reacted a day later by refusing to sell oil to India, which means the mullahs got the message.