Proportionality in War and Signalling
First, let me be clear that I am not talking about the proportionality as defined in international law. I'm speaking of the mass media or man-on-the-street understanding--roughly defined as "Let the punishment fit the crime." While there's a place for that understanding--it makes starting a war on a pretext untenable--it is also profoundly harmful to the conduct of war as a whole.
Only a very few wars in history were won by a pure military victory. The rest was determined by posturing, by signaling. By convincing the enemy that your strength and willingness to fight are greater than theirs, you can seek a negotiated settlement. However, when one side limits its military action in mis-guided proportionality, the signaling effect is lost. What one side considers the judicious use of force, the other side may interpret as weakness or timidity. If one side displays weakness and timidity, real or not, the other side would be tempted to continue the conflict, if for no other reason than the eventual negotiated settlement may be more advantageous. Thus judicious force can extend wars and cause all of the associated harms.
On the other hand, the much-derided "Shock and Awe" of the Iraq War shows the limits of signalling for signaling's sake. Since they understood the campaign's function, it had much less impact than it would have otherwise.