Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(When Allah showed them unto thee) O Muhammad (in your dream) on the Day of Badr (as few in number, and if He had shown them to thee as many, ye (Muslims) would have faltered) you would have been fearful (and would have quarrelled over the affair) over the question of war. (But Allah saved (you)) He decreed otherwise. (Lo! He knoweth what is in the breasts (of men)) what is in people's heart.
Remember, when God showed them to you in your dream, in your sleep, as few, and so you informed your companions of this, and they were delighted, and had He shown them to you as many, you would have faltered, shrunk in cowardice, and quarrelled, fallen into disagreement, over the matter, the matter of fighting; but God saved, you, from faltering and quarrelling. He knows that which is in the breasts, that which is in the hearts.
When God showed them to you, O heart, in the dream-state of the inactivity of the outer senses and the tranquility of the corporeal faculties, [showing them to you] as being of little worth and feeble in [their] state, and had He shown them to you as many, in the state of the attributes of the soul being prevalent, you would have faltered and quarrelled, over the matter of breaking it [the soul] and vanquishing it, as each one of you is attracted to a [different] direction; but God saved [you], from faltering and quarrelling by means of His strengthening and protection [of you].
Allah made each Group look few in the Eye of the Other
Mujahid said, "In a dream, Allah showed the Prophet the enemy as few. The Prophet conveyed this news to his Companions and their resolve strengthened.'' Similar was said by Ibn Ishaq and several others. Allah said,
وَلَوْ أَرَاكَهُمْ كَثِيراً لَّفَشِلْتُمْ
(If He had shown them to you as many, you would surely, have been discouraged,) you would have cowardly abstained from meeting them and fell in dispute among yourselves,
وَلَـكِنَّ اللَّهَ سَلَّمَ
(But Allah saved), from all this, when He made you see them as few,
إِنَّهُ عَلِيمٌ بِذَاتِ الصُّدُورِ
(Certainly, He is the All-Knower of that is in the breasts.) 8:43.
Allah knows what the heart and the inner-self conceal,
(And (remember) when you met, He showed them to you as few in your eyes) demonstrates Allah's compassion towards the believers. Allah made them see few disbelievers in their eyes, so that they would be encouraged and feel eager to meet them. Abu Ishaq As-Subai`i said, that Abu `Ubaydah said that `Abdullah bin Mas`ud said, "They were made to seem few in our eyes during Badr, so that I said to a man who was next to me, `Do you think they are seventy' He said, `Rather, they are a hundred.' However, when we captured one of them, we asked him and he said, `We were a thousand.''' Ibn Abi Hatim and Ibn Jarir recorded it. Allah said next,
وَيُقَلِّلُكُمْ فِى أَعْيُنِهِمْ
(and He made you appear as few in their eyes,) Allah said,
وَإِذْ يُرِيكُمُوهُمْ إِذِ الْتَقَيْتُمْ
(And (remember) when you met. He showed them to you...), He encouraged each of the two groups against the other, according to `Ikrimah, as recorded by Ibn Abi Hatim. This statement has a Sahih chain of narrators. Muhammad bin Ishaq said that Yahya bin `Abbad bin `Abdullah bin Az-Zubayr narrated to him that his father said about Allah's statement,
لِّيَقْضِيَ اللَّهُ أَمْراً كَانَ مَفْعُولاً
(so that Allah might accomplish a matter already ordained, ) "In order for the war to start between them, so that He would have revenge against those whom He decided to have revenge (pagans), and grant and complete His favor upon those He decided to grant favor to, His supporters.'' The meaning of this, is that Allah encouraged each group against the other and made them look few in each other's eyes, so that they were eager to meet them. This occurred before the battle started, but when it started and Allah supported the believers with a thousand angels in succession, the disbelieving group saw the believers double their number. Allah said,
(There has already been a sign for you (O Jews) in the two armies that met (in combat, the battle of Badr). One was fighting in the cause of Allah, and as for the other, (they) were disbelievers. They (disbelievers) saw them (believers) with their own eyes twice their number. And Allah supports with His aid whom He wills. Verily, in this is a lesson for those who understand.) 3:13
This is how we combine these two Ayat, and certainly, each one of them is true, all the thanks are due to Allah and all the favors are from Him.
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
This happened when the Holy Prophet was marching from Al Madinah towards the battlefield and it was not then known how large the army of the disbelievers was. In this dream, he saw the army and concluded from what he saw that it was not large. He related what he saw in his dream to the Muslims, and this filled them with courage and they marched on boldly towards the enemy.