This third item or party is often hidden, often overlooked. It is overlooked by both Mr. A and by Mr. B. It is overlooked by Nation G and Nation H. They are fighting visibly with each other because of a THIRD entity neither one sees.
Look over the history of the Peloponnesian Wars (in ancient Greece) and you see Sparta and Athens fighting with each other. Men tried to stop that war for decades. There was a third party in it. Persia.
It was the policy of Persia to prevent any Greek city from rising stronger than the rest. The Persian prince, Cyrus, exercised that policy by using Persian gold to bring agitation between Athens and Sparta and keep them at war.
On a more domestic basis, when a husband and wife quarrel continually there is usually a hidden third party to the quarrel.