On even the most narrow view of the inherent right of self-defence, the US was lawfully entitled to act against Soleimani/IRGC, especially in circumstances where Iran's proxies had attacked the US' Baghdad Embassy & Iran had 'form' in attacking US diplomatic & military personnel
I cannot believe that a proposition so obvious has to be stated but here we are. International law confers upon a nation not only a right of self defence against an armed attack but also against an attack that is imminent. No state is required to first suffer an attack.
The question of imminence is one that people will debate but a nation's right of anticipatory self-defence does not require specific time/place for enemy attack. Where your enemy is determined and mobile, the criteria of imminence becomes correspondingly more fluid than fixed.
One other aspect is proportionality. A strike that damages, relevantly, the imminent threat by decapitating its command element, is a military act of self-defence that is proportionate. Such proportionality falls well within the classic 'just war' understanding of self-defence.
Will add that pretty much all Western militaries - especially the Anglophone allies - have very mature processes for targeting & effecting military actions to further force-protection objectives. These processes pertain regardless of who, as a political matter, is in power.
