When we see a fight, the combatants are in such full view that we seldom look for the real instigator of the fight. It required a third party to cause it.
After a war man sometimes looks for the third party who agitated it. He could as easily look before the war began.
There is an odd phenomenon which lawyers know well. When two combatants suddenly find a third who privately has been urging each to fight, the combatants unite, and then shun the third party. There is a reverse proof that tends to prove this situation, also well known to lawyers. It requires a third party, a lawyer or arbitrator or judge, to settle quarrels. These are very visible. Well, apparently it takes a hidden third party to begin a quarrel in the first place!
Agitators, rumormongers, gossips, such people are the third parties to quarrels. On a large scale they have undisclosed employers who set them to work. On a small scale they are just troublemakers who themselves hate life. In the final analysis, two do NOT make a quarrel. It takes a hidden third.