The Quran

إِذ أَنتُم بِالعُدوَةِ الدُّنيا وَهُم بِالعُدوَةِ القُصوىٰ وَالرَّكبُ أَسفَلَ مِنكُم ۚ وَلَو تَواعَدتُم لَاختَلَفتُم فِي الميعادِ ۙ وَلٰكِن لِيَقضِيَ اللَّهُ أَمرًا كانَ مَفعولًا لِيَهلِكَ مَن هَلَكَ عَن بَيِّنَةٍ وَيَحيىٰ مَن حَيَّ عَن بَيِّنَةٍ ۗ وَإِنَّ اللَّهَ لَسَميعٌ عَليمٌ

Muhammad Asad

[Remember that day] when you were at the near end of the valley [of Badr], and they were at its farthest end, while the caravan was below you.¹ And if you had known that a battle was to take place, you would indeed have refused to accept the challenge:² but [the battle was brought about none the less,] so that God might accomplish a thing [which He willed] to be done,³ [and] that he who would perish might perish in clear evidence of the truth, and that he who would remain alive might live in clear evidence of the truth.⁴ And, behold, God is indeed all-hearing, all-knowing.

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).
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).