In Guns, Germs and Steel, Jared Diamond proposes an alternative explanation for the rise of the modern, organized state (government). He contrasts it with a traditional explanation for why states exist.

In a 1970 article, Robert Caneiro posited that states arise out of ecologically limiting conditions caused and perpetuated by warfare. When two societies clash and there is limited land, war is the result. Those wars make organization and leadership (the state) necessary.

Diamond has a longer view of human organization, and he sees the production of food as the key to understanding how and why humans create complex societies. Prior to the agricultural revolution about 12,000 years ago, humans existed in small hunter-gatherer tribes. Like our primate cousins, chimps and gorillas, the leadership of these small groups was one powerful (male) figure whom the rest of the tribe turned to and who set the rules. In small groups, this type of leadership is sufficient.

In humans, those leaders took on the "king" roles as societies became larger. But kings were only needed in large groups. What has made large groups possible, argues Diamond, is the efficient production of food (specifically high protein, stable sources such as wheat) which has had several effects.

First, a stable method of food production means people can stop traveling to find food sources; instead, they create permanent communities around their agricultural fields. Second, people can have more children because women are able to raise multiple children when they do not have to travel—women do not need to carry more than one child from place to place. Having more children means more hands on the farm and better food production, but it also means larger communities.

The result of all this food is more people. Large numbers of people, densely packed together, creates a need for social organization. The traditional tribe leader becomes a king who needs bureaucracy and hierarchy to function. Hence, the state is born. The state does organize the military, but war is not what leads to states.

The only mention of a person named Carneiro in Guns, Germs, and Steel comes in the notes to Chapter 14.  There, Diamond refers to a journal article from 1970 in which Robert Carneiro argues that (in Diamond’s words)

States arise through warfare under conditions in which land is ecologically limiting.

This theory is indeed contrary to what Diamond believes.  The main argument of this book as a whole is that agriculture is the key to world history.  Diamond believes that areas that were first to develop agriculture were the most likely to become powerful.  The creation of centralized states is part of this process of gaining power.

Diamond argues that agriculture leads to a need for centralized government.  As agriculture is adopted, higher population densities become possible.  When this happens, centralized governments are needed to control the larger populations.  As larger become more organized, more agriculture is possible.  This leads to still larger populations and, thereby, to more centralized governments.  As Diamond says on p. 286 in the paperback edition of the book,

Food production, which increases population size, also acts in many ways to make features of complex societies possible…

In this view, it is successful agriculture, and not conflict over scarce agricultural land, that causes centralized governments to arise.