Tafsir of Sad 38:13

أُولَـٰئِكَ الْأَحْزَابُ (They were the [ strong ] groups. - 38:13) One explanation of this statement is that it is the description of the word 'groups' in verse 11. In other words, 'these are the groups or Confederates that have been pointed out in this verse.' Maulana Thanavi (رح) has gone by this Tafsir in his explanation of the verse. But, other commentators explain it in the sense that actually they were the groups that were really strong, not these people of Makkah. In other words, those who possessed real strength and power were the people of Nuh and ` Ad and Thamud and others like them. Compared with them, the Mushrihs of Makkah were nothing. When such powerful people could not escape Divine punishment, they would hardly count. (Qurtubi)