I have read both Infanteer's and Steve's responses, as well as your post on the linked thread. My answer, I think, is complementary to all the above perspectives, perhaps coming from a slightly different angle. It's a good question, and one that has been bouncing around my head for the last few days since reading your thread.

I would suggest that the intensity of warfare refers to political will, in an explicitly Clausewitzian setting. If the political will is for complete and utter annihilation of another population group, then the violence will be total - witness the devastation visited upon Carthage or on the Eastern Front. If the political objectives desire a new balance of power then the need to co-exist post hostilities requires reduced levels of violence.

Political systems may sometimes topple themselves mid-conflict, which is why 'pauses' sometimes exist or the intensity of violence decreases. If the political system is unable to tolerate an ongoing offensive or military campaign due to materiel losses, economic cost, human cost or even parliamentary support, then policy may change, based on the omnipresent centre of gravity, political sustainability.

When the will for total destruction of another population is present as a political goal the violence will be absolute. When that political will decreases in intensity the level of violence, too, reduces. After all, states go to war with the intent of a better peace, so the means exerted will relate to their view and conception of the end-state.

This would explain why states involved in a civil war may often resort to extreme levels of violence as they do not expect to have to rehabilitate or coexist alongside the opposing political order. It would also explain why limitations are placed on even the most high-risk operations, such as the employment of gas in WW2 or nuclear weapons in the Cold War campaigns, as both sides intended to work with and not annihilate the opposing populations.

Perhaps I am rephrasing your question. I don't believe that the intensity of violence differs between time periods, but rather due to the objectives of politics. The trend for political systems to emphasise stability and the maintenance of a balance of power may have moved away from absolutes in conquest and destruction towards less costly and less permanent measures, and it is this phenomenon that is now observed in reduced levels and intensity of violence in warfare.