President Trump approved an offensive cyberstrike that disabled Iranian computer systems used to control rocket and missile launches, even as he backed away from a conventional military attack in response to its downing Thursday of an unmanned U.S. surveillance drone, according to people familiar with the matter. The cyberstrikes, launched Thursday night by personnel with U.S. Cyber Command, were in the works for weeks if not months, according to two of these people, who said the Pentagon proposed launching them after Iran’s alleged attacks on two oil tankers in the Gulf of Oman earlier this month.