If a wound should touch you - there has already touched the [opposing] people a wound similar to it. And these days [of varying conditions] We alternate among the people so that Allāh may make evident those who believe and [may] take to Himself from among you martyrs - and Allāh does not like the wrongdoers -
The Wisdom Behind the Losses Muslims Suffered During Uhud
Allah states to His believing servants who suffered losses in the battle of Uhud, including seventy dead,
قَدْ خَلَتْ مِن قَبْلِكُمْ سُنَنٌ
(137... Many similar ways (and mishaps of life) were faced before you), for the previous nations who followed their Prophets before you, they too suffered losses. However, the good end was theirs, and the ultimate defeat was for the disbelievers. This is why Allah said,
The second verse that follows gives consolation to Muslims from another angle. It has been said that if Muslims were wounded or hurt in that particular battle, so also were those fighting against them. If seventy Muslim men were martyred with many wounded, is it not that they had condemned an identical number of their enemies to Hell, and wounded many, a year ago? Then, in this very battle, many a men from the enemy ranks were killed and wounded initially. So, when the text says:
If you receive a wound, they have received a similar wound. And such days we rotate among the people..
It guides us to another important principle and rule of procedure.
In this mortal world, the customary practice of Allah Almighty is to cause the days of hardship and ease, pain and comfort, suffering and peace occur among people by turns. If, for some reason, a falsely-motivated power succeeds in getting a short-lived upper hand, the group motivated by the truth should not lose heart and come to think that, from this point onwards, they are always doomed to nothing but defeat. Instead of taking this negative attitude, they should rather go about finding out the causes of that defeat, and once they have discovered those, they should take corrective measures and eliminate all possibilities of repeating those mistakes. In the end, the group motivated by the truth shall emerge as the ultimate victor.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 138–140
Those who fear God lead their lives as if God is watching over them, that is, as if all their actions are performed so as to be more and more pleasing in the eyes of God. Instead of leading a life of licentiousness, they lead a life of constraint. The requirements of the religion of God become their own requirements and for that they spend in charity under all circumstances, irrespective of whether they have much or little. When enraged, they control themselves. When there is a cause for complaint, they forgive instead of reacting negatively. They too fall into error, but that is only temporary. They immediately realize their mistake and return to God, spontaneously invoking His forgiveness and seeking His mercy. Whatever has been stated verbally in the Quran is borne out by the events of Islamic history. But advice is heeded only by those who are keen to have it.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.