Hilf al-Fudhul war had shown the residents of Makkah that they lacked a central authority. They also realized Makkah was largely unjust. After their expeditions to Yemen and Syria they came to conclusion that the element of justice was surely mussing in Makkah. The justice was to be provided to everyone the natives and the travelers, and the wanderers alike.
4 of the 10 Quraish tribes formed the League. They were Banu Hashim, led by al-Zubair uncle of the Holy Prophet (P.B.U.H), Banu Asad, Banu Zubrah, and Banu Taym.
The Holy Prophet (P.B.U.H) who was present at the oath taking ceremony was clearly happy with its just aims. Later in life, he upheld the oath and conceded its validity. He said “… if I be invited to such a pact again I will surly take part.”
This shows that justice is a universal principle. Islam accepts justice even if Muslims aren’t involved.
It is pertinent to note here that these 4 clans were poor compared to the others who didn’t join.