Surah Al-Anfaal (8) — Ayah 66

Now Allah has lightened your burden, for He knows that there is weakness [69] in you. So if there are a hundred steadfast among you, they will overcome two hundred, and if there are a thousand, they will, by Allah’s permission, overcome two thousand. And Allah is with those who are steadfast.

[69]
The Reason for Later Weakness Among Muslims is Not the Abrogation of the Previous Command, But Circumstances:

Although this verse and the previous verse have been placed together, there is a considerable gap between their times of revelation, and they have only been placed together due to the relevance of their subject matter. The time of revelation of the first verse is after the Battle of Badr, when most of this surah had already been revealed. As for the time of revelation of the second verse, nothing can be said with certainty, but it appears to be from the time of the Battle of Hunayn or after it. Those who became Muslim at the time of the conquest of Makkah did not possess the same courage as those who had accepted Islam from the beginning, and there could be several reasons for this weakness. For example, one reason is that Muslims were initially in the minority, and a minority, for the sake of its survival, must remain much more alert and courageous than the majority. Secondly, the way the Muhajirun and the early Ansar were trained, after some time, many of them became old and weak, and some were martyred or passed away. Those who became new Muslims or the new generation that emerged did not possess the same insight, steadfastness, and courage as the earlier Muhajirun and Ansar. Furthermore, with an increase in numbers, reliance on Allah tends to decrease, or people become boastful about their numbers, which is a trait of human nature. Therefore, when the command that one man should prevail over ten became burdensome for such people, Allah lightened this command. However, this does not mean that the first command has now been abrogated or is no longer applicable. Rather, it means that when these new Muslims are fully trained and their faith is fully strengthened, then the previous command will again be applicable. Indeed, this actually happened in the last battles of the Prophetic era. In one battle, one thousand Muslims faced eighty thousand disbelievers, and at the Battle of Mu'tah, three thousand Muslims stood firm against one hundred thousand disbelievers, and this ratio is even greater than one to ten. Then, during the era of the Rightly Guided Caliphs, there are also many such examples. However, from this verse of concession, the scholars have deduced the ruling that if the number of disbelievers is double or less, then in such a case, fleeing from battle is forbidden. In any case, Muslims relying solely on the verse of concession leads to a decrease in their patience and endurance, as is evident from the following hadith.

The Negative Result of the Concession:

Sayyiduna Ibn Abbas ؓ says: When this verse was revealed that twenty steadfast among you should prevail over two hundred, it was burdensome for the Muslims. Allah had made it obligatory upon the believers that one Muslim should not flee from ten disbelievers. Then Allah granted concession in this verse and said that at the very least, one hundred should certainly prevail over two hundred. Thus, when Allah reduced the numbers, the patience among the Muslims also decreased accordingly. [بخاري۔ كتاب التفسير]
See the commentary of this verse along with the next verse.