US Defense Secretary Donald H. Rumsfeld made his strongest public case for a plan, opposed by some in the US Congress and by Russia, to convert some Navy long-range missiles from a nuclear to a conventional role for potential use against terrorist targets anywhere in the world. Rumsfeld held talks Sunday with Russian Defense Minister Sergei Ivanov, and they discussed the issue at a joint news conference. Opponents of the plan argue that it could create a situation in which a conventionally armed US Trident missile, launched from a submarine, would be mistaken for a nuclear launch, thus risking the possibility of a retaliatory nuclear strike. Rumsfeld said he thought little of that argument. He said the Pentagon would be "fully transparent" with Moscow about any such conversion of strategic missiles, so that there was no room for miscalculation. Ivanov, however, made clear that his government opposes the plan. The Russian defense chief said he understands that Rumsfeld sees this prospective weapon as a way of maximizing US options for "preventive strikes," meaning attacks against terrorist targets that are launched not in response to a terrorist act but in order to destroy a terrorist weapon before it can be used.