A while ago it was fashionable to demand that war should only be considered as last resort in foreign politics. I never really got into this idea, and it was opposed by the hawks at least behind the scenes.
It was an arbitrary maxim that did not stand plausibility and theoretical tests - nor did it stand the utter lack of principles and self-discipline amongst Western foreign politicians who increasingly bought into the idea that they could 'intervene' militarily in distant places at little political cost.
Why is violence only as last resort wrong?
Well, this idea that violence is justified only if nothing else helps is misplaced. Violence is justified if it's the least terrible choice.
It makes no sense to stay peaceful and endure a predictably worse outcome than by fighting back. On the other hand, it makes no sense to fight (back) and endure a worse outcome than achievable by diplomacy.
Most importantly - and this is what hawks don't seem to get - there's extremely rarely (if ever nowadays) an opportunity to actually gain something by being violent.
Violence / warfare is inherently destructive, not productive. You could only gain a material advantage by stealing. You can gain immaterial advantages (such as national independence or freedom in civil war) by becoming violent, of course.
In the end, the choice between war and peace depends a lot on preferences. How highly do you rate the losses of war, how highly do you rate the advantages of achieving political objectives in war, how highly do you estimate the probability of achieving them through warfare in relation to diplomacy?
It's understandable that people disagree on the question of waging war or not.
It's also perfectly understandable that Western hawks a.k.a. warmongers want to invade country x, then y, bomb z and tell us that this is serious, good foreign policy: They are idiots. Idiots have idiotic ideas.