Abu'l-Najm Badr ibn Abdallah al-Jamali al-Mustansiri, better known as Badr al-Jamali (Arabic: بدر الجمالى) or by his eventual title as Amir al-Juyush (أمير الجيوش, lit. 'Commander of the Armies'), was a military commander and statesman for the Fatimid Caliphate under Caliph al-Mustansir. Of Armenian origin but a convert to Islam, Badr had been brought up as a military slave (mamluk) by the ruler of Tripoli, Jamal al-Dawla ibn Ammar. In the 1060s, he was appointed twice as governor of Damascus in Syria, at a time when Fatimid authority there was disintegrating, and the central government in Egypt was on the verge of collapse as a result of the Mustansirite Hardship. Badr was unable to prevent the loss of most of Syria to local potentates and Turkoman warlords, but managed to hold on to the coastal cities, making Acre his base.

As the sole major military commander outside Egypt, he was called upon by al-Mustansir to help rescue the dynasty. Badr landed in Egypt in late 1073, rapidly eliminated his rivals, and was appointed vizier with plenipotentiary powers, making him a quasi-sultan or military dictator, with the caliph relegated to his religious duties as Isma'ili imam. Control over the state was solidified with the marriage of one of his daughters to al-Mustansir, and the appointment of his own son, al-Afdal Shahanshah, as his successor in 1085. Badr managed to restore order in Egypt and initiated major administrative reforms, defeated Turkoman attempts to invade Egypt, and recovered control over Palestine and the Hejaz. He initiated a series of new constructions, including the Juyushi Mosque and the new city wall of Cairo, some of whose gates stand to this day. Badr's tenure saved the Fatimid regime, but also began a period where the vizierate was dominated by military strongmen who held power on their own, rather than through caliphal appointment, and who increasingly sidelined the caliphs to puppet rulers. Badr also initiated a wave of Armenian migration into Egypt, and was the first of a series of viziers of Armenian origin, who played a major role in the fortunes of the Fatimid Caliphate over the subsequent century.

Badr was of Armenian ethnic origin, and born sometime between 1005 and 1008; he is recorded as being over 80 years of age at the time of his death. As his patroymic 'ibn Abdallah' indicates, he was most likely born into a Christian family and later converted to Islam. He was purchased as a military slave (mamluk or ghulam) by Jamal al-Dawla ibn Ammar, ruler of Tripoli, whence he acquired his epithet of 'al-Jamali'. His name, Badr, meaning 'full moon', is likewise typical for slaves. Otherwise his early life and career until c. 1063 are obscure. However, the historian Seta Dadoyan suggests that he may be identifiable with a namesake Abu'l-Najm Badr, a young Armenian ghulam who briefly ruled over Aleppo on behalf of the Fatimids in 1022. At some point, Badr married the daughter of Ruqtash, a Turkoman officer in the Fatimid army.

Badr's career begins to be documented in April 1063, when he was appointed military governor of Damascus and its province in succession to al-Mu'ayyad Mu'tazz al-Dawla Haydara ibn al-Husayn. Badr is recorded at this point as bearing the honorific titles of 'Crown of Commanders' (Taj al-Umara), 'Commander of the Armies' (Muqaddam al-Juyush), and 'Honour of the Realm' (Sharaf al-Mulk). He made Mizza near Damascus his residence. According to Thierry Bianquis, Badr may have tried to exploit local rivalries to strengthen his position, by choosing as fiscal administrator Yahya ibn Zayd al-Zaydi, who hailed from one of the two powerful Husaynid ashraf families of the city, the other being the Banu Abi'l-Jinn family, that had previously held the post, along with that of qadi of Damascus.

Badr's tenure was cut short little more than a year later, in June/July 1064, after clashes between his troops and the local people and the urban militia (ahdath). Two other governors, Haydara al-Kutami and Durri al-Mustansiri, followed in short succession, but by the end of the year Damascus was left without governor for several months. The troubles in Syria were symptomatic of the wider crisis afflicting the Fatimid state, which during this period neared complete collapse. The weak rule of Caliph al-Mustansir (r. 1036–1094) had opened the central government to intrigues and rivalries; the vizierate was held by favourites of the Caliph's mother, Rasad, changing hands over thirty times between 1063–1067; and factional infighting between the different ethnic contingents of the Fatimid army broke out, crippling the administration and exhausting the treasury. Open clashes broke out in Cairo between the Turks and the black African troops (Sudan) favoured by Rasad in 1062 and again in 1067, when the Turks under Nasir al-Dawla Ibn Hamdan seized control of the capital and expelled their rivals to Upper Egypt.

On 3 July 1066, Badr returned to Damascus, not only as its governor, but of all of Syria, with the additional honorific of 'Sword of Islam' (Sayf al-Islam), and took possession of the Qasr al-Saltana fortress, located just outside the city walls, as his seat. Within a few days he learned of the death of his son, Sha'ban, at Acre.

In the meantime, Badr's second tenure in Damascus proved as troubled as the first, facing uprisings by the ahdath, whose motives are unclear. His persecution of the Banu Abi'l-Jinn is recorded, as well as the imprisonment or exile of several of the city's notables. Among the exiles was Abu Tahir Haydara ibn Ibrahim Ibn Abi'l-Jinn, who served as naqib al-ashraf (dean of the Alids) in Damascus. Abu Tahir went to Cairo to lodge complaint with Nasir al-Dawla Ibn Hamdan, who then held power in Egypt and was a declared rival of Badr. Ibn Hamdan tried to enlist Abu Tahir and the Bedouin tribes of the Syrian Desert in the cause of eliminating Badr; the latter had opposed the Bedouin tribes (Banu Tayy, Banu Kalb, and Banu Qays) who, taking advantage of the weakening of Fatimid authority, tried to seize lands in the Hawran Plain and Jordan River valley. Ibn Hamdan released Humayd and Hazim, two imprisoned members of the Banu'l-Jarrah, the leading family of the Tayy, gave them 40,000 gold dinars to kill Badr, and sent Abu Tahir along with them to Syria.

Badr was also unable to halt the collapse of the Fatimid state's authority in the region, specifically the Emirate of Aleppo. The chief city of northern Syria, Aleppo had become a Fatimid vassal c. 1009. After that, periods of direct Fatimid rule alternated with rule by the local Mirdasid dynasty; the last Fatimid attempt to recover direct control of the city was defeated in 1060 and marked the start of Fatimid decline in the region, although the Mirdasids continued to recognize nominal Fatimid suzerainty. Factinal conflict between members of the Mirdasid clan continued to plague Aleppo, however, until in 1065, Mahmud ibn Mirdas seized the city. In 1067, according to the 13th-century Damascene historian Sibt ibn al-Jawzi, Mahmud refused a demand by Caliph al-Mustansir (r. 1036–1094) to resume raids against the Byzantine Empire, render tribute and expel the Turkoman warrior band under Ibn Khan that had helped him gain Aleppo. Badr was instructed to bring Mahmud to heel, and turned to the latter's deposed uncle, Atiyya ibn Salih, promising him assistance to regain control of Aleppo. Atiyya marched on Aleppo, but the two Mirdasids were reconciled by Ibn Ammar of Tripoli, dividing the emirate between themselves and confirming their nominal allegiance to the Fatimid caliph. When Atiyya went to Damascus to join Badr, however, his stronghold, Rahba, was seized by the Uqaylid emir of Mosul, Muslim ibn Quraysh, who had the Friday prayer read in the name of the Abbasid caliph.