There is a agreement among scholars that these verses were revealed concerning the battle of Uhud (Tafsir al-Jalalayn, Tanwir al-Miqbas min Tafsir Ibn ‘Abbas, Asbab Al-Nuzul by Al-Wahidi and Tafsir Ibn Kathir)
Previously, having written about the Battle of Uhud, this conflict took place as a consequence of the Quraysh declaring war against the Muslims, because the enemy had lost at the Battle of Badr, 1400 years ago. They yearned for revenge and under the command of Abu Sufyan they prepared for war against Muslims.
3:121 And [remember] when you, [O Muhammad], left your family in the morning to post the believers at their stations for the battle [of Uhud] – and Allah is Hearing and Knowing –
3:122 When two parties among you were about to lose courage, but Allah was their ally; and upon Allah the believers should rely.
3:123 And already had Allah given you victory at [the battle of] Badr while you were few in number. Then fear Allah ; perhaps you will be grateful.
3:124 [Remember] when you said to the believers, “Is it not sufficient for you that your Lord should reinforce you with three thousand angels sent down?
3:125 Yes, if you remain patient and conscious of Allah and the enemy come upon you [attacking] in rage, your Lord will reinforce you with five thousand angels having marks [of distinction]
3:126 And Allah made it not except as [a sign of] good tidings for you and to reassure your hearts thereby. And victory is not except from Allah, the Exalted in Might, the Wise –
3:121 – Tells us that Prophet Mohammed (p) left his family behind at home so to station his companions, ready to engage the enemy. Notice, the verse doesn’t say that Muhammed went to the enemy’s area. Rather, it tells us the enemy came to their land.
3:122 – This verse (3:122) informs us that some of the parties among the Muslims were about to lose courage. Lose courage may mean here, that some may have feared the enemy, and wanted to abandon defending themselves i.e., some of whom wanted to run away from the enemy.
3:123 – This says that Muslims at the battle of Badr were few when they went up against the enemy who were three more than the Muslims.
3: 124 – from the verse it hints to us that some of whom who were on the side of Prophet Muhammed (p) were weak in faith. And the Prophet (p) reminds them that indeed God will ‘reinforce’ and help the Muslims in this battle with 3000 Angels against the enemy Quraysh.
3:125 – Here we are told, from reading the verse that if the Muslims remained patient and remember God, and if the enemies were to attack them in ‘rage’, that God would help them by sending Angels against the attackers.
This verse is remarkable in understanding the situation the Muslims were in at the battle of Uhud. It us shows that some of the Muslims feared, thought that they would be attacked unawares. From the verses we gather that the Muslims were not the aggressors. It was the Quraysh whom started war against the Muslims.
“90 This reference to the battle of Uhud, to which many verses of this surah are devoted, connects with the exhortation implied in the preceding verse, “if you are patient in adversity and conscious of God, their guile cannot harm you at all”. Since this and the subsequent references cannot be fully understood without a knowledge of the historical background, a brief account of the battle would seem to be indicated. In order to avenge their catastrophic defeat at Badr in the second year after the hijrah, the pagan Meccans – supported by several tribes hostile to the Muslims – mustered in the following year an army comprising ten thousand men under the command of Abu Sufyan and marched against Medina. On hearing of their approach, in the month of Shawwal 3 H., the Prophet held a council of war at which the tactics to be adopted were discussed. In view of the overwhelming cavalry forces at the disposal of the enemy, the Prophet himself was of the opinion that the Muslims should give battle from behind the fortifications of Medina and, if need be, fight in its narrow streets and lanes; and his plan was supported by some of the most outstanding among his Companions. However, the majority of the Muslim leaders who participated in the council strongly insisted on going forth and meeting the enemy in the open field. In obedience to the Qur’anic principle that all communal affairs must be transacted on the basis of mutually-agreed decisions (see verse 159 of this surah, as well as 42:38), the Prophet sorrowfully gave way to the will of the majority and set out with his followers towards the plain below the mountain of Uhud, a little over three miles from Medina. His army consisted of less than one thousand men; but on the way to Mount Uhud this number was still further reduced by the defection of some three hundred men led by the hypocritical ‘Abd Allah ibn Ubayy, who pretended to be convinced that the Muslims did not really intend to fight. Shortly before the battle, two other groups from among the Prophet’s forces – namely, the clans of Banu Salamah (of the tribe of Al-Aws) and Banu Harithah (of the tribe of Khazraj) almost lost heart and were about to join the defectors (3:122) on the plea that because of their numerical weakness the Muslims must now avoid giving battle; but at the last moment they decided to follow the Prophet. Having less than seven hundred men with him, the Prophet arrayed the bulk of his forces with their backs to the mountain and posted all his archers – numbering fifty – on a nearby hill in order to provide cover against an outflanking manoeuvre by the enemy cavalry; these archers were ordered not to leave their post under any circumstances. In their subsequent, death-defying assault upon the greatly superior forces of the pagan Quraysh, the Muslims gained a decisive advantage over the former and almost routed them. At that moment, however, most of the archers, believing that the battle had been won and fearing lest they lose their share of the spoils, abandoned their covering position and joined the melee around the encampment of the Quraysh. Seizing this opportunity, the bulk of the Meccan cavalry under the command of Khalid ibn al-Walid (who shortly after this battle embraced Islam and later became one of the greatest Muslim generals of all times) veered round in a wide arc and attacked the Muslim forces from the rear. Deprived of the cover of the archers, and caught between two fires, the Muslims retreated in disorder, with the loss of many lives. The Prophet himself and a handful of his most stalwart Companions defended themselves desperately; and the Prophet was seriously injured and fell to the ground. The cry immediately arose, “The Apostle of God has been killed!” Many of the Muslims began to flee; some among them were even prepared to throw themselves upon the mercy of the enemy. But a few of the Companions – among them ‘Umar ibn al-Khattab and Talhah – called out, “What good are your lives without him, O believers? Let us die as he has died!” – and threw themselves with the strength of despair against the Meccans. Their example at once found an echo among the rest of the Muslims, who in the meantime had learnt that the Prophet was alive: they rallied and counter-attacked the enemy, and thus saved the day. But the Muslims were now too exhausted to exploit their chances of victory, and the battle ended in a draw, with the enemy retreating in the direction of Mecca. On the next day the Prophet started in pursuit of them at the head of seventy of his Companions. But when the Muslims reached the place called Hamra’ al-Asad, about eight miles south of Medina, it became obvious that the Meccans were in no mood to risk another encounter and were rapidly marching home; and thereupon the tiny Muslim army returned to Medina.
91 I.e., the clans of Banu Salamah and Banu Harithah, who had almost joined the deserters led by ‘Abd Allah ibn Ubayy (see preceding note).
92 A reference to the battle of Badr, in 2 H., which is dealt with extensively in surah 8.” 
Tafhim al-Qur’an – Sayyid Abul Ala Maududi:
“94 From here begins the fourth discourse, which was revealed after the battle of Uhd. It has been beautifully connected with the previous discourse which ended with the admonition that the machinations of their enemies could do them no harm, if they would show restraint, and fear Allah. This discourse is an apt continuation of the same theme, because the setback in the battle of Uhd was due to lack of restraint and lack of fear of Allah.
As a review has been made of all the important events of the battle in order to bring home the lessons, it will be very useful to keep in view their background, because in the discourse only passing references have been made to them without any details.
In the beginning of Shawwal (A.H. 3), the Quraish attacked Al-Madinah with an army of 3,000 men. They not only out-numbered the Muslims but were also much better equipped. Besides this, they were filled with passion to avenge their defeat at Badr. The Holy Prophet and his experienced Companions were of the opinion that they should defend Al-Madinah from within the walls, but some young men, who had not taken part in the battle of Badr, were so imbued with the spirit of martyrdom that they insisted on going out to meet the enemy. At last on their persistence, the Holy Prophet decided to go out to fight. So he led out an army of 1,000 men but at Shaut, `Abdullah bin Ubayy, the ring leader of the hypocrites, deserted him with his 300 men. Naturally this treacherous act produced much confusion in the Muslim army. So much so that two parties, Banu Salmah and Banu Harithah, were so disheartened that they began to waver in their minds and were about to turn back, but were dissuaded from this by those Companions who had remained firm and constant.
Then the Holy Prophet marched to Mount Uhd with the 700 men left with him after the desertion of `Abdullah bin Ubayy. He arrayed his men at the foot of the Mount so that the army of the Quraish was in their front and Mount Uhd at their back. Thus the only vulnerable point was a pass on one side, where he posted 50 archers under the command of `Abdullah bin Jubair, saying, ” to let not any enemy come near us and do not leave your posts in any case whatsoever. Even if you see birds peck at our flesh, you should not abandon your posts at all.” 
Malik Ghulam Farid:
“467. The reference is to the Battle of Uhud. In order to wipe out the humiliation of their defeat at Badr, the Quraish of Mecca, in the third year of the Hijrah marched against Medina with a well-equipped army of 3,000 seasoned warriors. Much against his own wish the Holy Prophet with a force of 1,000 including 300 followers of Abd Allah b. Ubayy, the notorious hypocrite who afterwards defected, marched out of Medina to meet the enemy. The encounter took place near Uhud.
468. The two parties were the two tribes of Banu Salimah and Banu Harithah, belonging respectively to Khazraj and Aus (Bukhari, kitab al-Maghazi). The verse indicates that they did not actually show cowardice, but seeing that by defection of Abd Allah’s 300 followers, the small Muslim army had been further greatly depleted, they only thought of deserting but in fact did not do so.” 
Maulana Muhammad Ali:
“121a. It is to the events of the battle of Uhud that this and the following sections are devoted. In the third year of the Hijrah Abu Sufyan marched against Madinah. The Prophet at first intended to stay within the town, but afterwards marched into the open field with a thousand men, one-third of whom, under the leadership of Abd Allah ibn Ubayy, the great hypocrite leader, left him and returned to Madinah. The enemy were first completely routed, but fifty Muslim archers, who were placed in a strong position to cut off the retreat of the enemy, made a mistake, and in order to join in the pursuit left their position. The enemy fell back upon the Muslims who were now in disorder and had lost their naturally fortified position, and after inflicting some loss upon them, left the field secure from the pursuit of the Muslim force. It was not a victory for the Quraish, who thought it safe to go back when they found the Muslims involved in their own troubles. They could not take a single prisoner of war, nor had they the courage to attack Madinah, which they did two years later with a very strong force.
122a. the two tribes of the Banu Salimah, and the Banu Harithah are meant (B. 64:18).
122b. This shows that they did not actually show cowardice. The desertion of the Muslim force by Abd Allah ibn Ubayy with his three hundred men made some of the Muslims also entertain the idea of deserting the army on account of superior enemy forces, but they did not actually desert it.” 
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 The Message of The Quran translated and explained by Muhammad Asad, page 135 – 137
 Sayyid Abul Ala Maududi – Tafhim al-Qur’an – The Meaning of the Qur’an – http://www.englishtafsir.com/Quran/3/index.html#sdfootnote95sym and http://www.searchtruth.com/tafsir/tafsir.php?chapter=3
 The Holy Qur’an – Arabic Text With English Translation & Short Commentary By Malik Ghulam Farid, page 156
 The Holy Quran Arabic Text with English Translation, Commentary and comprehensive Introduction [Year 2002 Edition] by Maulana Muhammad Ali, page 170