Since the end of WWII, and in particular since the end of the Cold War, there has been a decrease in interstate wars (between states) and a noticeable increase in intrastate wars (within states). Why has this been the case?
Since the end of WWII and the Cold War, there has been a marked reduction in interstate wars and a significant increase in intrastate wars because of a multitude of reasons.
The reduction in interstate wars can be attributed to two major reasons: the United Nations and technological developments. The presence of the United Nations ensures that interstate conflicts are, to the maximum possible extent, resolved by negotiations and dialogues. The UN can also authorize peace-keeping forces to ensure that conflicts are taken care of. Technological developments, such as high-tech surveillance, news media and most importantly nuclear deterrence has also ensured that interstate conflicts are resolved by non-military means.
Intrastate affairs, on the other hand, have worsened, especially in some parts of the world. These conflicts are typically due to some small religious or autocratic group who fights against the state for autonomy. This conflict may not necessarily be focused on overthrowing the country, but center instead on regional autonomy. Some groups such as the Taliban or ISIS may also have transnational ambitions by appealing to people of a particular religion or descent. Technological innovations and global connectivity has aided these small groups, and in some cases, neighboring countries also help these small intrastate players (such as Pakistan's involvement in India).