Don't know about who exactly are the 'leftists' you are referring to but here's my take :
Palestine : Dont send bombs. Send Aid. Lift blockade so Palestinians dont suffer.
Iran : Dont send bombs. Send Aid. Lift sanctions so Iranian people dont suffer.
Interested to hear your take regarding the same.
And why should liberal countries trade with genocidal regimes, so that they don't kill their own people? Is that seriously what you're proposing - appease the bully?
Lifting sanctions just helps the mullahs flex their power on Iranian civilians. Lifted sanctions means more suffering for Iranian people and people abroad suffering from Iran-funded terrorist groups.