New Delhi, July 11 -- JAKARTA - When Indonesia signed an agreement to acquire India's BrahMos supersonic cruise missiles in Jakarta on Monday, the ceremony was brief and the language formal. The strategic signal underneath was neither. The deal, worth roughly $600 million and completed during Prime Minister Narendra Modi's three-day state visit, made Indonesia only the third country in the world to purchase the BrahMos system and the first in Southeast Asia to pair it with an air force that still relies on Russian-built fighters.

President Prabowo Subianto called it a "historic milestone." The description carried weight. Indonesia operates Su-27SK and Su-30MK jets sourced from Russia, platforms whose armament has historically come from M...