This verse was revealed concerning the expedition to Tabuk (Tafsir ibn Kathir and Ibn Juzayy).
For more information why the expedition of Tabuk took place, please read the following article for its historical context: “Answering Jihad: ‘Fight Against Those Who Do Not Believe’ – Quran 9:29“.
“And it is not for the believers to go forth [to battle] all at once. For there should separate from every division of them a group [remaining] to obtain understanding in the religion and warn their people when they return to them that they might be cautious.” – Quran 9:122
9:122 – God orders the Muslims who were going to the expedition of Tabuk, not to go forth to the battle all at once. There should be some left at home to teach others on matters of religion (Tafsir ibn Kathir and Ibn Juzayy).
Asbab Al-Nuzul by Al-Wahidi (d. 1075 AD):
(And the believers should not all go out to fight…) [9:122]. Al-Kalbi reported that Ibn ‘Abbas said: “When Allah, exalted is He, revealed the defects of the hypocrites due to their failure to join Jihad, the believers said: ‘By Allah, we will never fail to join any military conquest or expedition undertaken by the Messenger of Allah’. So when the Messenger of Allah, ordered the march of military expeditions to fight the enemy, all the Muslims went out to fight and left the Messenger of Allah, alone in Medina. Allah, exalted is He, then revealed this verse”. (Asbab Al-Nuzul by Al-Wahidi on Surah 9:122 – Online Source)
Tafsir Ibn Kathir (1301 – 1373 AD)
“Allah the Exalted here explains His order to Muslims to march forth with the Messenger of Allah for the battle of Tabuk. We should first mention that a group of the Salaf said that marching along with the Messenger , when he went to battle, was at first obliged on all Muslims, because, as they say, Allah said, ‘March forth, whether you are light or heavy’ 9:41, and,…” (Tafsir Ibn Kathir on Surah 9:122 – Online Source)
Dr. Muhammad Asad:
“162 Lit., “admonish their people when they come back to them, so that they might be on their guard”. Although the above injunction mentions specifically religious knowledge, it has a positive bearing on every kind of knowledge – and this in view of the fact that the Qur’an does not draw any dividing-line between the spiritual and the worldly concerns of life but, rather, regards them as different aspects of one and the same reality. In many of its verses, the Qur’an calls upon the believer to observe all nature and to discern God’s creative activity in its manifold phenomena and “laws”, as well as to mediate upon the lessons of history with a view to gaining a deeper insight into man’s motivations and the innermost springs of his behaviour: and, thus, the Qur’an itself is characterized as addressed to “those who think”. In short, intellectual activity as such is postulated as a valid way to a better understanding of God’s will and – if pursued with moral consciousness – as a valid method of worshipping God. This Qur’anic principle has been emphasized in many well-authenticated sayings of the Prophet, for instance, “Striving after knowledge is a sacred duty (faridah) for every man and woman who has surrendered himself or herself to God (muslim wa-muslimah)” (Ibn Majah); or, “The superiority (fadl) of a learned man over a [mere] worshipper [i.e., one who merely prays, fasts, etc.] is like the superiority of the full moon over all the stars” (Tirmidhi, Abu Da’ud, Ibn Majah, Ibn Hanbal, Darimi). Consequently, the obligation of the believers to “devote themselves to acquiring a deeper knowledge of the Faith” (li-yatafaqqahu fi’d-din) and to impart its results to their fellow-believers relates to every branch of knowledge as well as to its practical application.” 
Maulana Muhammad Ali:
“122a. The introduction here of the subject of the study of religion shows the aim which the Qur’an has in view. In the midst of the ordinances relating to fighting, it introduces the subject of the preparation of a missionary force, thus showing that this was the greatest necessity of Islam. It was only by means of a missionary effort that truth could be spread, and the ultimate object was not to be lost sight of, even when the community was engaged in a life-and-death struggle against overwhelming forces of the enemy.” 
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 The Message of The Quran translated and explained by Muhammad Asad, page 411
 The Holy Quran Arabic Text with English Translation, Commentary and comprehensive Introduction [Year 2002 Edition] by Maulana Muhammad Ali, page 429