Say you are tribe A, warring tribe B.
If everyone likes tribe A, they will help, or at least not hinder you. If everyone hates tribe A, or loves tribe B, people will hinder you, whether by attacking, or supporting the enemy.
I recall an example of this from W11, a small tribe PROT, of only 10 or so members, and under 1mil points total, went to war with a 75 million point tribe. They survived over a year, and because of their PnP presence, by the middle of the war, were getting support from almost every other tribe, and were gaining good ground. Sadly, after over a year of constant attacking, and RL issues, more than half the tribe quit.