I am not a pacifist but I do abhor war. History teaches that we must defend ourselves from aggression and nations are entitled to do that and when they do that they must pursue their defence with all possible means that accord to the rules of war and are consistent with accepted civilised conduct. The activities of ISIS in Syria and Iraq are considered by many world leaders to be a threat to many nations and those leaders believe that threat extends to acts of terrorism carried out in places other than Syria and Iraq.
If those leaders are right, then it seems to me that bombing in Iraq and Syria does not constitute an adequate defence of their nations. It has the further disadvantage of driving many of those bombed into the arms of ISIS, which strike me as being led by psychopaths. Bombing defends neither the people of Syria and Iraq to any significant degree, nor does it protect other nations from acts of terrorism.
The logical conclusion is that ISIS must be fought and defeated by all military and other means available, even if it does mean aligning temporarily with undesirable tyrants in order to do this. This would mean in effect, sending an army in to defeat ISIS, forming an alliance with president Assad of Syria, and defeating ISIS permanently, bringing their leaders to trial under international law.
There have been many historical precedents which show that democratic nations have aligned with autocratic dictators in order to defeat a common enemy, and many historical precedents which show the lack of effectiveness of bombing. You cannot fight a war with bombs alone.
It also strikes me that it should not be beyond the wit of the great democracies to cut off the supply of money and arms to ISIS. It seems that no effective attempt has been made to do this.