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This is a very free rendering of the elliptic phrase which runs, literally, thus: "And if you had mutually made an appointment, you would indeed have failed to keep the appointment" - i.e., for battle. As already mentioned in the introductory note to this surah, most of the Prophet's followers had been under the impression that their objective was the relatively weak trade caravan, and some of them were dismayed at finding themselves face to face with the powerful Quraysh army advancing from the south.
According to all the commentators, the words interpolated by me between brackets are implied in this highly elliptical sentence. Literally translated, its last words might be rendered as "a thing that was [already] done": meaning that if God decrees a thing, it must inevitably come about, and may therefore be described as already done.
Some of the great commentators understand this sentence in a metaphorical sense, with "destruction" signifying persistence in denying the truth (kufr), and "life" being synonymous with faith. According to this interpretation, the above sentence would have the following meaning: "... so that the denial of the truth on the part of him who has denied it, and the faith of him who has attained to it, might become clearly evident" (Zamakhshari); or "let him who is bent on denying the truth go on denying it after this clear evidence of God's will, and let him who has attained to belief go on believing" (Ibn Ishaq, as quoted by Ibn Kathir). In my opinion, however, it is preferable to interpret the references to death and life in their prima-facie (that is, not metaphorical) sense - namely, as applying to the death or survival of all who took part in the battle of Badr, believers and unbelievers alike: the believers who fell in that battle died conscious of being martyrs in God's cause, and those who survived could now clearly discern God's hand in their victory; while the dead among the deniers of the truth had clearly given their lives for nothing, and those of them who survived must now realize that their crushing defeat was due, in the last resort, to something infinitely greater than the valour of the Muslims (cf. verse {17}, and the corresponding notes).
Before the beginning of the battle, the Prophet and his followers were encamped in the northern part of the valley of Badr, nearest to Medina, while their enemies, having come from Mecca, occupied its southern part. The Meccan trade caravan, coming from Syria under the leadership of Abu Sufyan, was in the meantime proceeding southwards through the coastal lowlands (see introductory note to this surah).
The battle was not planned since the Muslim force was after the caravan. Had the two armies made plans to meet in battle at that time and place, they both probably would have missed each other.
The little Islamic force from Madinah went out to meet the big Makkan army, and they met on the two sides of a valley at Badr, while the Quraish caravan was on lower ground towards the sea, about 3 miles from Badr.
They were all at cross purposes. The caravan was making for Makkah, but scarcely thought it could get there. The Quraish force was trying to save the caravan and then annihilate the Muslims. The Muslims had decided to attack the Quraish army from Makkah which turned out to be big, more than three times their number. Yet the two forces met, precisely at the spot and at the time when a decisive battle should take place and the Muslims dispose of the pretensions of the Makkans. If they had carefully planned a mutual appointment, they could not have carried it out more precisely. On the Muslim side the few martyrs knew that the victory was theirs and those who survived the battle enjoyed the fruits of the victory. On the pagan side, both those who died and those who lived knew fully the issue joined. Even psychologically both sides went in with full determination to decide the issue.