and to the orphans, and to the poor and to the wayfarer, if ye believe in God and in that which we have sent down to our servant on the day of victory,1 the day of the meeting of the hosts. Sura Al-Anfal (viii) 42.

This is the law of Islam on the subject to the present day.

The victory of Badr was needed to strengthen Muhammad's position in Madina, which had now become. much weakened by the small success of his previous warlike expeditions and no efforts were now spared to attribute this great success to miraculous intervention on the part of God. Thus:—

When ye sought succour of your Lord, He answered you, 'I will verily aid you with a thousand angels, rank on rank.'2 And God made this promise as pure good tidings, and to assure your hearts by it;

1 All commentators refer this to the victory at Badr called the يَومَ الفُرقَانِ . This phrase is translated as 'day of grace,' 'day of victory,' 'day of destruction,' 'day of Badr in which was the separation of the good from the evil'—
روز بدركة جدا شدن حق أز باطل در و بود
Rabbi Geiger shows that فرقان is derived from the Rabbinical Hebrew word פדקז and means 'deliverance,' 'redemption,' and applies it in Sura Al-Baqarah (ii) 181 to the month of Ramadan, as the month of deliverance from sin and not, as is usually done, to the Qur'an, as that which illuminates and distinguishes.
See Geiger, Judaism and Islam (S.P.C.K. Madras), p. 41, and Rodwell's translation of the Qur'an, p: 176. Note ii.
Husain interprets the word
فرقان thus:—
الفُرقَانَ ـ أز حدود وسائر شرائع دين كة جدا كنندة است ميان حق وباطل
that is, 'The laws, regulations and all the laws of religion which separate the good from the evil'—Tafsir-i-Husaini, vol. i, p. 30.
2 In a later Sura this number grew to three thousand—Sura Al-'Imran (iii) 120.

for succour cometh from God alone.' Sura Al-Anfal (viii) 9-10.

Later on, this aid is referred to as a support to the Prophet's claim and as an encouragement and a warning to his followers.

Ye have already had a sign,2 in the meeting of the two hosts. The one host fought in the cause of God and the other was infidel. To their own eyesight the infidels saw you twice as many as themselves,3 and God aids with His succour whom He wishes. And in this there is indeed a warning to those who have perception. Sura Al-'Imran (iii) 11.

It was not ye who slew them, but God slew them, and the shafts were God's, not thine. Sura Al-Anfal (viii) 17.

Those who gave up their lives are spoken of as martyrs for God's cause.

Say not of those who are slain on God's path that they are dead: nay, they are living. Sura Al-Baqarah (ii) 149.4

1 Baidawi says that it is uncertain whether this is addressed to the Quraish, or the Jews or the believers in Madina. 'Abbas says it refers to the men of Mecca or of Madina. 
2 That is, in the battle of Badr there was a clear sign of the prophetship of Muhammad. Thus Husain says of the words —
كَانَ لَكُم آيةً —'To you was a sign'—that they mean—
شما را علامتى ونشائى درست بر نبوتِ محمد
'To you was a mark and good sign of the prophetship of Muhammad'. Tafsir-i-Husaini, vol. i, p. 71.
3 In Sura Al-Anfal (viii) 46, the Muslims were said to be diminished in the eyes of the Meccans. The commentators admit the discrepancy and try to reconcile the statements by making the statement in Sura Al-'Imran (iii) 11, succeed the one in Sura Al-Anfal (viii) 46. See Wherry on this subject, Commentary, vol. ii, p. 7.
وَ لاَ تُقُولُوا لمَن يٌقْتَلُ فِى سَبيلِ ألّّهِ أمْوَتًا بَل أحياء
It is uncertain whether this verse refers to the battle of Badr or of Uhud. The commentator Husain says:—
دور روز بدر جان شيرين بداد واز نعمت حيات ولذت نصيم دنيا محروم شد
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