The Quran

وَإِذ يَعِدُكُمُ اللَّهُ إِحدَى الطّائِفَتَينِ أَنَّها لَكُم وَتَوَدّونَ أَنَّ غَيرَ ذاتِ الشَّوكَةِ تَكونُ لَكُم وَيُريدُ اللَّهُ أَن يُحِقَّ الحَقَّ بِكَلِماتِهِ وَيَقطَعَ دابِرَ الكافِرينَ

Muhammad Asad

And, lo, God gave you the promise that one of the two [enemy] hosts would fall to you: and you would have liked to seize the less powerful one,¹ whereas it was God's will to prove the truth to be true in accordance with His words, and to wipe out the last remnant of those who denied the truth²-

Lit., "while you would have liked the one which was not powerful to be yours" - i.e., the caravan coming from Syria, which was accompanied by only forty armed men and could therefore, be attacked without great danger.
The destruction of the Meccan army at Badr was the prelude to the elimination, in the course of the next few years, of all opposition to Islam in its homeland: and it is to this future fulfilment of God's promise that the above words refer. See also surah 11, note 103.