Why might two countries become enemies?

Sadly, there are so many reasons for two countries to become enemies.  They can disagree about a territory, for example, as China asserts that it "owns" some areas of the South China Sea.  Similarly, they can disagree about where a border lies between the two countries.  They can become enemies because of some other enmity one engages in, for example, as a country might become our enemy if it attacks a nation we are sworn to protect under the North Atlantic Treaty Organization (NATO), a kind of "the enemy of a friend is our enemy" situation. Russia could become our enemy quite easily like this.  They can become enemies over trade policy, if one country imposes tariffs on or an embargo against goods from the other country.  They can become enemies over other kinds of policies, as we consider North Korea to be an enemy because of its repressive Communist regime and its pursuit of nuclear advancement in weaponry.  Countries can become enemies over natural resources, if one country seeks to use the natural resources of another country without its agreement.  The only saving grace in today's world is that because of nuclear proliferation and a modern tendency, at least in the western world, to try to rely more on diplomacy, being enemies does not always lead inevitably to war.