Quds force commander, Qassim Soleimani death by drone strike approved by President Trump is legal. That decision was not a decision to go to war or launch additional military conflict(s) with Iran. How about referring to General David Petraeus confirming that killing Soleimani is more significant and consequential that taking out Osama bin Ladin and al Baghdadi.
Former Department of Homeland Security Secretary and lawyer, Jeh Johnson also confirmed the order/approval to kill Soleimani by President Trump is legal without Congressional knowledge or approval.