Al-Ma'mun 1 is called Abdelas (Ἀβδελᾶς) (Abdullah) in Theophanes Confessor, and Ismael (τῷ Ἰσμαήλ) or Mamoun in Theophanes Continuatus. His name was `Abd Allah al-Ma'mun: Chron. §192 (II, p. 7). He was a son of the caliph Harun 1 and younger brother of Mouamed (al-Amin 1): Theoph. AM 6301 (Ἀβδελᾶς, ὁ ἀδελφὸς αὐτοῦ), Theoph. AM 6304 (τῷ ἀδελφῷ αὐτοῦ Ἀβδελᾷ). Son of the caliph Harun 1 al-Rashid, he was the younger brother of Muhammad (al-Amin 1) and older brother of al-Qasim 1: Chron. 1234, §192 (II, p. 7), §194 (II, p. 8). Brother also of Abu Ishaq (i.e. al-Mu`tasim 1): Chron. 1234, §213 (II, p. 24), §215 (II, p. 27). Father of Abbas 1: Chron. 1234, §213 (II, p. 24), §215 (II, p. 27). Harun 1 arranged that at his death he should be succeeded by his eldest son, Muhammad (al-Amin 1), who would then be succeeded by the other two brothers in turn in order of seniority; Al- Ma'mun 1 was meanwhile given authority over Khorasan; however Muhammad (al-Amin 1) assumed power and seized treasures belonging to al-Ma'mun 1 and also tried to secure the succession for his own son; a three-year civil war followed between al-Ma'mun 1 and al-Amin 1, ending with the defeat and death (in AH 197, i.e. AD 812/813) of al-Amin 1; al-Ma'mun 1 then became caliph, settling in Baghdad: Chron. 1234, §§192, 194 (II, pp. 8-9), §198 (II, p. 10). When Harun 1 died in March 809 al-Amin 1 succeeded as caliph but Abdelas (al-Ma'mun 1) revolted from his base in Khorasan, employing his father's troops, and there ensued five years of civil war: Theoph. AM 6301. In 812 he defeated his brother in the East (τὴν ἐνδοτέραν Περσικὴν): Theoph. AM 6304 (his brother fled and seized Baghdad). He was caliph from 813 to 833. He was the ruler of Syria (τὸν τῆς Συριας ἄρχοντα), living in Baghdad, whom Ioannes 5 visited as ambassador early in the reign of the emperor Theophilos 5; he is several times styled ὁ ἀμεραμνουνῆς: Theoph. Cont. III 9 (pp. 96-98), Zon. XV 26. 19-30. The Persian Babak 1 was in rebellion against him (ὁ ἀμεραμνουνῆς, unnamed): Theoph. Cont. III 21. He is the unnamed caliph whom Manuel 1 is said to have served under with success, subduing Khorasan to his rule: Theoph. Cont. III 25 (pp. 118-119), Zon. XV 28. 19-21. The caliph, with whom Iannes (Ioannes 5) negotiated a peace, at the same time secretly persuading Manuel 1 to return to the emperor: Genesius III 10. He agreed to Manuel 1's proposal to lead an Arab army against the Romans with his son, when Manuel 1 secretly planned to return to Constantinople, in c. 830: Theoph. Cont. III 26 (p. 120) (ὁ ἀμεραμνουνῆς, called Ismael), Zon. XV 28. 25-27. Early in the reign of the emperor Theophilos 5 he prepared a major invasion of Roman territory with the help of Manuel 1; he left Baghdad with a huge army "in year 1140 Sel., which was AH 214" (i.e. AD 829), "along with the rebel Emmanuel the patrikios", who, having deserted the Romans, promised to help al-Ma'mun 1 conquer the Roman empire; the invasion took place in AH 215 (i.e. AD 830), when the Arabs captured many fortresses and towns; these included the town of Semalouos, which was destroyed and whose inhabitants were all carried away into captivity after the fighting men had been killed; those towns which submitted were treated leniently and placed by al-Ma'mun 1 under the control of Manuel 1; al-Ma'mun 1 then returned to Baghdad: Chron. 1234, §§210-212 (II, pp. 22- 24).

In the following year (is this 831 or 832?) Manuel 1 betrayed him, and al-Ma'mun 1 led another army, with his son Abbas 1 and his brother Abu Ishaq (i.e. al-Mu`tasim 1), into Roman territory and again took many towns and fortresses; he spent the following winter at Kayshum: Chron. 1234, §213 (II, pp. 24-25). Then in the following year (114 Sel., i.e. AD 833) al-Ma'mun 1 invaded Roman territory for the third time, sending troops ahead under Abbas 1 and Abu Ishaq (al-Mu`tasim 1); al-Ma'mun 1 ordered the rebuilding of the city of Tyana, but while the work was in progress he fell ill and died; the date was Wednesday, 23rd of Tammuz, in AH 218 (i.e. 23 July 833): Chron. 1234, §§215-216 (II, pp. 27-28).

He was greatly interested in secular studies in general and, supposedly, in geometry in particular (ἄλλοις τε μαθήμασι σχολάζων Ἑλληνικοῖς καὶ δὴ καὶ γεωμετρίας ὡς ἐπὶ τὸ πλεῖστον οὐκ ἀμελῶν); learning of the great knowledge which Leo 19 had of the subject (cf. Manikophagos 1), he tried to persuade him to leave the Roman empire and come to him to teach him what he knew: Theoph. Cont. IV 27 (pp. 186ff.), cf. Zon. XVI 4. 14-22 (unnamed). For his scholarly interests, see also Chron. 1234, §192 (II, p. 7) ("this al-Ma'mun was a wise and eloquent man, skilled in the science of astronomy and grammar. He composed and wrote a marvellous book about the science of astronomy and up to today it is well known among astronomers").

He was also interested in the beliefs of the Christians and famously debated on this topic with the bishop of Harran, Theodore Abu Qurra (Theodoros 322); their debate was recorded in a book "for whoever wishes to read it": Chron. 1234, §211 (II, p. 23).

Brother of Amin (al-Amin 1), whom he succeeded as caliph in AH 198 (Sept. 813/Aug. 814; probably Sept./Oct. 813, as al-Amin was killed on the fifth of Muharram, i. e. very early in AH 198): Bar Hebr., p. 126. In the year 1133 or 1134 Sel. (AD 821/823) he sent Taoma (Thomas 7) with an army against Constantinople: Bar Hebr., p. 129. Early in the reign of Theophilos 5, he was encouraged by the fugitive Manuel 1 to invade the lands of the Romans; in the month of Haziran (June) he invaded and captured four fortresses in Cappadocia; then in the following Iyar (May) of the year 1142 Sel. (May 831) he again invaded and besieged the fortress of Lolon (Loulon) but without success, until the following year when the starving inhabitants agreed to surrender Ujair; Manuel 1 then chose to return to the empire and al-Ma'mun 1 threatened to crush the Romans; Theophilos 5 made overtures for peace, but al-Ma'mun 1's terms, the submission of the empire to him, were not acceptable, and al-Ma'mun 1 invaded Cilicia; there he caused an impostor who claimed to be of imperial family to be crowned emperor by the Chalcedonian patriarch Iob (Iob 1); in the year 1143 Sel., in the month Iyar (May) he invaded the lands of the Romans with his sons al-Mu`tasim 1 and Abbas 1 and made preparations for the rebuilding of Tyana; there he fell ill and died and was buried at Tarsos in Cilicia; the date of his death was "the fourth day of the week (Wednesday), on the eighth day of the fourth month of the two hundred and eighteenth year" (of the Hegira), i.e. April/May 833; he was succeeded by his son (Abu Ishaq), al-Mu`tasim 1: Bar Hebr., pp. 132-133. Abdullah (al-Ma'mun 1) was the son of Aaron (Harun al-Rashid; Harun 1) and the brother of Muhammad (al-Amin 1); civil war broke out between him and his brother after their father's death: Const. Porph., DAI 22, 65ff.

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