[There have been] more than 300 drone strikes and some 2,500 people killed by the Central Intelligence Agency and the military since Mr. Obama first took office. Mr. Obama and his advisers are still debating whether remote-control killing should be a measure of last resort against imminent threats to the United States, or a more flexible tool, available to help allied governments attack their enemies or to prevent militants from controlling territory. Though publicly the administration presents a united front on the use of drones, behind the scenes there is longstanding tension. The administration is still pushing to make the rules formal and resolve internal uncertainty and disagreement about exactly when lethal action is justified. The Defense Department and the C.I.A. continue to press for greater latitude to carry out strikes. The administration's legal reasoning has not persuaded many other countries that the strikes are acceptable under international law. For years before the Sept. 11, 2001, attacks, the United States routinely condemned targeted killings of suspected terrorists by Israel, and most countries still object to such measures. Partly because United Nations officials know that the United States is setting a legal and ethical precedent for other countries developing armed drones, the U.N. plans to open a unit in Geneva early next year to investigate American drone strikes.