Operation Trident and its follow-up Operation Python were naval offensive operations launched on Pakistan’s port city of Karachi by the Indian Navy during the Indo-Pakistani War of 1971. Operation Trident resulted in the first use of anti-ship missiles in combat in the region,  and only the second worldwide (the first being the sinking of the Eilat) as well as the first sinking of naval vessels during hostilities in the region since World War II.India celebrates its Navy Day annually on 4 December to mark this operation.

Source: google