Although the history of South Arabia in the fifth and sixth centuries has not been studied to any great extent, the events of this period were of marked importance in the history of South Arabia in particular and of Central Arabia in general. Within this period we find the enlargement of the South Arabian kings' title, and the extension of their sovereignty over the highland of West and Central Arabia; the Abyssinian invasion of South Arabia and the semi-independent government of Abraha and his sons; and, finally, the abolition of South Arabian independence after the Persian invasion. South Arabia lost its position as the leading power in the Arabian peninsula said became a vassal province of the Persian empire. Moreover, the decline of Kinda and its migration to South Arabia created instability in Central Arabia, and indeed most of ayyām al-'arab [Days of the Arab] which we know, belongs to the period after the decline of Kinda, the vassal of South Arabia. The aim of the present work is to study this period of the history of South Arabia from the time of Abū Karib As'sad, who had the title "King of sb'/wdrydn/wḥḍrmwt/wymnt/w'rb/ṭwd/wthmt." 1. Chapter I deals with the relations of South and Central Arabia before the reign of Abū Karib As'sad; the extension of South Arabia towards the north; the eventual conquest of Kinda; the expedition of Sharafddin's Inscription towards the land of Tanūkh and Persia, at the time of Shammar Yuhar'ish; and finally the counter-attack of Shapur II, King of Persia and Imru'l-Kais. 2. Chapter II treats of the reign of Abū Karib As'sad; the enlarged title, ''rb, twd and thmt; Abū Karib in Central Arabia; the legends of Abū Karib's invasion of Irak and Central Asia, the siege of Madina, and finally his worshipping at the Ka'ba in Mecca. 3. Chapter III deals with the traditional kings after Abū Karib who have been mentioned in the inscriptions, with special reference to Hassan's expedition against Diadis, 'Abd Kulal in Arab tradition, and Ma'dikarib Ya'fur in Central Arabia. 4. Chapter IV covers the reign of the famous king Yūsuf 'As'ar, his origin, and the massacre of the Christians in South Arabia. 5. Chapter V is concerned with the Abyssinian invasion of South Arabia, the battlefield, the period of the puppet king Sumyafa Ashwa, and his end at the hand of the famous King Abraha. 6. Chapter VI deals with the most significant achievements of Abrah; the events of CIH 541; the events of Ry 506; the expedition of Ry 506 and its relation to the expedition of the elephant; the reign of Abraha's sons, Yaksūm and Masrūk; and finally the end of the Abyssinian domination of South Arabia.