and Eastern Palestine to the Greeks, the annexation of
the Arabians involved the extension of the war beyond the limits of
Arabia to a struggle with the two great powers (see further ARABIA:
_History_).

After the subjugation of middle and north-eastern Arabia, Khalid b.
al-Walid proceeded by order of the caliph to the conquest of the
districts on the lower Euphrates. Thence he was summoned to Syria, where
hostilities had also broken out. Damascus fell late in the summer of
635, and on the 20th of August 636 was fought the great decisive battle
on the Hieromax (Yarmuk), which caused the emperor Heraclius (q.v.)
finally to abandon Syria.[3] Left to themselves, the Christians
henceforward defended themselves only in isolated cases in the fortified
cities; for the most part they witnessed the disappearance of the
Byzantine power without regret. Meanwhile the war was also carried on
against the Persians in Irak, unsuccessfully at first, until the tide
turned at the battle of Kadisiya (Kadessia, Qadisiya) (end of 637). In
consequence of the defeat which they here sustained, the Persians were
forced to abandon the western portion of their empire and limit
themselves to Iran proper. The Moslems made themselves masters of
Ctesiphon (Madain), the residence of the Sassanids on the Tigris, and
conquered in the immediately following years the country of the two
rivers. In 639 the armies of Syria and Irak were face to face in
Mesopotamia. In a short time they had taken from the Aryans all the
principal old Semitic lands--Palestine, Syria, Mesopotamia, Assyria and
Babylonia. To these was soon added Egypt, which was overrun with little
difficulty by 'Amr ibn-el-Ass (q.v.) in 640. (See EGYPT: _History_, S
Mahommedan.) This completed the circle of the lands bordering on the
wilderness of Arabia; within these limits annexation was practicable and
natural, a repetition indeed of what had often previously occurred. The
kingdoms of Ghassan and Hira, advanced posts hitherto, now became the
headquarters of the Arabs; the new empire had its centres on the one
hand at Damascus, on the other hand at Kufa and Basra, the two
newly-founded cities in the region of old Babylonia. The capital of
Islam continued indeed for a while to be Medina, but soon the Hejaz
(Hijaz) and the whole of Arabia proper lay quite on the outskirt of
affairs.

The ease with which the native populations of the conquered districts,
exclusively or prevailingly Christian, a