As the aerial contest between Hamas (rockets) and Israel (air strikes) rages, it would be good to remember a number of things.
One, just because civilians die does not make it a war crime. It depends on who you are shooting at and how.
If you are aiming at civilians--as the 400+ Hamas rocket launches are--that is a war crime.
If you are aiming at military targets--as the Israelis are--it may not be a war crime if the civilian deaths are incidental to the military purpose.
For example, if you are taking sniper fire from a building, carpet bombing the entire city to kill that sniper is a war crime.
Using a couple artillery rounds even if one is off target and kills a civilian is not a war crime.
Further, you may be responsible for committing a war crime if an enemy kills your own civilians if you put your military assets close to civilians for use as human shields.
And forget about that "proportionality" nonsense. Military force is about achieving an objective and not "fair" casualties.
If someone attacks you, you've used too little force if your attacker isn't defeated.
No rule of war requires you to respond with ineffective force when attacked.