Within a war - this is often shaped by political and social conditions as well as military necessity and/or realities. Violence can taper, as Infanteer mentioned, because both sides need a breather. It can also be because the political leadership of nation X decides to tone down violence to send a signal of some sort. Likewise it can ratchet back up due to domestic pressure on political leaders in response to something that happened (or didn't happen) on the battlefield. The appearance of a "cause" can also cause violence levels to increase (sometimes drastically and quickly).

Between wars I think the same sorts of things apply. Nations exhausted by a major conflict don't seem to have the same stomach for aggression (at least at first), or they may feel that their demands/needs have been met. Political unrest at home can also lead to diminishing violence outside the borders as leaders turn their attention inward. In "ye olden dayes" armies had to be smaller due to logistics limits and obviously couldn't be moved as quickly. This mean that nations or rulers had to "pick their time and place" in a different way than they do now. It's worth considering the impact of social changes on this as well. The rise of communications technology (from printing on) and nation-states and (I think) the dominance of monotheistic religion also play roles.