The Imminent Threat

Four days ago a U.S. drone strike killed the head of Iran's Quds forces, General Qasem Soleimani. That has set off a chain of events that we have no idea where it's going to lead. How is the United States government justifying this? Is this an AUMF type situation or has that been phased out? And are we seeing what Benjamin Whittis was worrying about more than five years ago that it's some kind of imminent threat type justification?