India, Sept. 17 -- Buried in the diplomatic language of India's protest over a warship collision in the North Arabian Sea is a reference that matters more than it might first appear: Article 10 of the Agreement between India and Pakistan on Advance Notice on Military Exercises, Manoeuvres and Troop Movements, signed in April 1991. It is worth understanding exactly what that provision says, and how routinely Pakistan's Navy appears willing to disregard it.
Article 10 exists because the high seas, unlike land borders, have no visible line separating one country's warships from another's. Naval vessels and submarines from both countries routinely operate in the same broad expanse of the Arabian Sea and its approaches. To prevent an accidental...