There are a couple of principles here that people need to get there heads round.
The concept that there are 'Rules of War' is complete nonsense, a delusion of gentlemen soldiers from the 19th century who had this romantic notion that war could be conducted along the lines of a cricket match, and you could roll out the stumps, play a bit, declare a winner, shake hands and all go home for tea and crumpets in the Summerhouse.
War, as I wrote elsewhere, is not the opposite of Peace. Peace has rules and morals and ethics. War cannot have such a thing, because war isn't civilised, and so lacks those facets of civilisations that make them peaceful, i.e. morals and ethics. Something as utterly uncivilised as war cannot have 'rules of conduct', because it goes contrary to the only objective of war, which is to win, and to win you hit harder, longer, and quicker than your opponent, because in the end, the only thing that actually matters about war is that you win, because very few societies survive intact after losing a war.
The ratio of civilian casualties to military causalities has risen in every war since the Wars of the Roses. This was initially due to the increasingly more destructive and sophisticated types of weaponry, which culminated in the Area Bombing Campaign of the RAF and the use of the Atomic Bomb on Japan. At that point a turning point occurred and two things happened - ever more sophisticated military hardware started to make war safer for soldiers, and it was realised that to defeat an enemy, not only was it it necessary to destroy the other sides forces, but it was also necessary to destroy the other sides industrial and social capacity to make war
As someone else pointed out, bombing civilians for the purpose of breaking morale doesn't work - the Germans discovered this in the Blitz, the English failed to learn that and tried the same thing in the Area Bombing campaign, and even the Americans failed to take note and tried it in Vietnam with exactly the same results
Thus, the nature of modern war, and the nature of modern method
s of making war have turned war from a battle between opposing armed forces to a contest between opposing societies. War itself is about to take two further twists in the next war. The increasing dependency of modern societies on information means that the most successful way to destroy a modern opponent will be an attack on the country's information systems by computer viruses, satellite destruction and attacks on the economy, for example by impairing the ability of a nation to trade on the worlds markets by systematic destruction of trading and currency bodies
The second development is the rise of Islam as a serious threat to western cultures. Whereas western cultures separate religion, government and the military, and each part functions separately, Islam is a complete all in one enemy. It's not possible to conduct a war against an Islamic society without waging war against the peoples of that society. This is because the inability of Islam to tolerate the existence of any other form of ruling body, military, religious or civil, means it's an all or nothing war - a winner takes all approach. Add this to the prolific use in Islamic societies of civilian suicide bombers makes it impossible to separate civilian from military from fanatic.