According to the majority of scholars, these Ayat are describing the battle of Uhud, as Ibn 'Abbas, Al-Hasan, Qatadah, As-Suddi and others said. The battle of Uhud occurred on a Saturday, in the month of Shawwal, and Allah knows best.

The Messenger of Allah marched with a thousand of his Companions. When they reached the Shawt area, 'Abdullah bin Ubayy went back to Al-Madinah with a third of the army, claiming he was angry the Prophet did not listen to his advice. He and his supporters said, "If we knew that you would fight today, we would have accompanied you. However, we do not think that you will fight today." The Messenger of Allah marched until he reached the hillside in the area of Uhud, where they camped in the valley with Mount Uhud behind them. The Messenger of Allah said,

"No one starts fighting until I issue the command to fight."

The Messenger prepared his forces for battle, and his army was seven hundred men. He appointed 'Abdullah bin Jubayr, from Bani 'Amr bin 'Awf, to lead the archers who were fifty men. The Prophet said to them,

"Keep the horsemen away from us, and be aware that we might be attacked from your direction. If victory was for or against us, remain in your positions. And even if you see us being picked up by birds, do not abandon your positions."

The Prophet wore two protective shields and gave the flag to Mus'ab bin 'Umayr of Bani 'Abd Ad-Dar. The Prophet also allowed some young men to participate in fighting, but not others, whom he allowed to participate in the battle of Al-Khandaq two years later. The Quraysh mobilized their forces of three thousand men with two hundred horsemen on each flank. They appointed Khalid bin Al-Walid to lead the right side of the horsemen and 'Ikrimah Ibn Abi Jahl on the left side. They also gave their grand flag to the tribe of Bani 'Abd Ad-Dar. Allah willing, we will mention the details of this battle later on, if Allah wills. Allah said here,

And (remember) when you left your household in the morning to post the believers at their stations for the battle [3:121],

designating them to various positions, dividing the army to the left and right sides and placing them wherever you command them.

And Allah is All-Hearer, All-Knower, He hears what you say and knows what you conceal in your hearts. Allah said next,

When two parties from among you were about to lose heart, [3:122].

Al-Bukhari recorded that Jabir bin 'Abdullah said, "The Ayah,

When two parties from among you were about to lose heart

was revealed about us, [the two Muslim tribes of] Bani Harithah and Bani Salamah. I (or we) would not be pleased if it was not revealed, because Allah said in it,

but Allah was their Wali (Supporter and Protector) [3:122]."

Muslim recorded this Hadith from Sufyan bin 'Uyaynah.

Reminding the Believers of Their Victory at Badr

Allah said,

And Allah has already made you victorious at Badr, [3:123]
meaning, during the battle of Badr, which occurred on a Friday, the seventeenth of Ramadan, in the second year of Hijrah.

The day of Badr is known as Yawm Al-Furqan [the Day of the Clarification], by which Allah gave victory and dominance to Islam and its people and disgraced and destroyed Shirk, even though the Muslims were few. The Muslims numbered three hundred and thirteen men, with two horses and seventy camels. The rest were foot soldiers without enough supplies for the battle. The enemy army consisted of nine hundred to a thousand men, having enough shields and supplies, battle-ready horses and even various adornments.

However, Allah gave victory to His Messenger, supported His revelation, and illuminated success on the faces of the Prophet and his following. Allah also brought disgrace to Shayatan and his army. This is why Allah reminded His believing servants and pious party of this favor,

And Allah has already made you victorious at Badr, when you were a weak little force,

when you were few then. This Ayah reminds them that victory is only from Allah, not because of a large army and adequate supplies. This is why Allah said in another Ayah,

...and on the day of Hunayn (battle) when you rejoiced at your great number, but it availed you naught [9:25], until,

And Allah is Oft-Forgiving, Most Merciful [9:27].

Badr is an area between Makkah and Al-Madinah and is known by the well that bears its name, which in turn was so named after Badr bin An-Narayn, the person who dug the well.

So have Taqwa of Allah that you may be grateful. [3:123],
means, fulfill the obligations of His obedience.