the question stands. what is the difference between bombing civilian population to achieve military success and car-bombing civilian population, with a much lesser casualty rate?

I can think of many key differences. But the biggest is that one involves a nation state and the other involves a terrorist organization.

If you disagree with the actions of nation states, there's a very clear way to stop them.

Your argument could be easily turned into -- what is the difference between criminals and soldiers?  The answer: Responsibility.

Yes. The United States went around the world and toppled every regime that was hostile to it.

the URSS were still there, right?

But the biggest is that one involves a nation state and the other involves a terrorist organization.

Touché. Still, what if the nation-state of a designed territory hasn't the support of the population, and the population fights back (like French's resistance, but that is 1 example). If both sides attacks civilian targets that are supportive of their ennemies, why would one be more legitimate than another?

Your argument could be easily turned into -- what is the difference between criminals and soldiers? The answer: Responsibility.

Sadly, except for U.S. modern troopers (and many other modern country), responsibility isn't part of a regular soldier quota in a lot of country. And even for one of the country with one of the most morale army there is (yes, U.S.A.) (except for antipersonal mines and uses of some illegal metal combustion), there are still many criminal acts commited by your soldiers, which gives you a lot of bad press.

"Responsability" is not achieved. So, except for the "legitimation" of the organisation, I do not see any real difference between a U.S. soldier and an AlQaeda guerrila. Everyone has it's unfair advantage over the other.