Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(And when He made you (Muslims), when ye met (them)) on the Day of Badr, (see them with your eyes as few) such that He emboldened you vis-à-vis them, (and lessened you in their eyes) such that they were emboldened vis-à-vis you, ((it was) that Allah might conclude a thing) so that Allah might give victory and the spoils of war to the Prophet (pbuh) and his Companions and bring about death and defeat for Abu Jahl and his host (that must be done) that has to be. (Unto Allah all things) the end results of things (are brought back) in the Hereafter.
And when God made you see them, O believers, when you met — in your eyes as few, as 70 or 100, when they were [in fact] 1000, so that you would advance against them; and He made you seem as few in their eyes, so that they would advance and not turn back from fighting you — this was before close combat had begun, but when it began, He made them [the disbelievers] see these [believers] as twice their number, as stated in sūrat Āl ‘Imrān [Q. 3:13] — so that God might conclude a matter that was to be done; and to God all things are returned, reverting.
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.