The ship was an unarmed vessel on its way to a goodwill visit to Sri Lanka and coming from an international maritime exercise hosted by India, which the United States also attended and participated in. The US torpedoed it, and when it sank, the US did not apparently attempt to rescue any of the Dena's crew. Fortunately, Sri Lanka showed up and saved 30 people.

Mind you, the details of war are not always clear. The US says that the ship was armed, and it also says that they did make an effort to rescue the crew. The US does not explain why it failed to actually rescue anybody, of course.