In a male the protection of his livelihood is the strongest of all instincts. Without it he has no social level in any working group and, without this status ne female is going to choose him. No female has any method of comparison when judging his worthiness to become a parent — of being offered sex, his biggest enjoyment, voluntarily and at no cost — at no immediate cost anyway!
Because of this, the protection of a group for its continued existence — whatever else it may be doing at the time — is never far from foremost in its collective mind. As groups become larger — say to the typical sizes of nation-states — the protective power grows to huge proportions because it has a far wider choice of exceptionally forceful individuals to become leaders. Thus all nation-states are in a state of extreme wariness concerning the foreign policies of others — particularly (for traditional reasons) those geographically closest to it.
In each nation-state, anything that another nation-state may be doing that impinges, however marginally, on its own well being arouses an inevitable response, or at least a building up of frustration that may be added to any future slight. This means that every nation-state is at least on the edge of war and, as often as not, actually at war. Moreover, either condition is welcome to, if not sought by, leaders of governments because it adds to their personal profile and sense of power.
Nowadays, the major nations cannot afford to go to war with one another in a physical way with large armies, only against much smaller and weaker entities. Besides, the methods of warfare now available to major nation-states are mutually destructive. But all major nation-states, will have multifarious sectors within their economies so there will always be many policies that may jar on others — so many, in fact, that there will always be a minority in which they have different policies.
For example, Britain and America have different bank regulations and corporate taxation. They watch each others’ changes like hawks. Although these seem to be trivial when compared with out-and-out military warfare, they are two of the factors that, over the longer term, leads to exactly the same result — the raising or the lowering of the economic power of the countries concerned, their relative standing in the global league table and the relative happiness of their people.