Yahya led the Almoravid armies in their first campaigns, including captures of Sijilmassa and Awdaghost in 1054/55, but was himself killed in battle against a dissident Berber faction in the Adrar.
After their conversion to Islam during the 9th century, the Sanhaja desert tribes were united and, with the zeal of neophyte converts, launched a series of campaigns (jihad) against the "Sudanese" (pagan black peoples of sub-Saharan Africa).
After the Sanhaja union collapsed, most of their old dominions - particularly the citadels, caravan stops and oases on the lucrative trans-Saharan trade routes - were lost to the Ghana Empire to the south, and to the Zenata Maghrawa rulers of Sijilmassa to the north.
The opportunity seemed to arise in the late 1040s, after the death of Yahya ibn Ibrahim, the chieftain of the neighboring Godala tribe, and high chief of the Sanhaja confederation.
Although only one of several candidates, Yahya ibn Umar succeeded in being elected the new high Sanhaja chief, a selection which provoked the resentment of the Godala, who had hoped for one of their own.
Invoking stories of the early life of Muhammad, Ibn Yasin preached that conquest was a necessary addendum to Islamicization, that it was not enough merely to adhere to God's law, but necessary to also destroy opposition to it.
[4] Fired up by religious fervor, in the early 1050s, the Lamtuna launched a series of campaigns against the neighboring tribes to persuade them - by force if need be - to join the new Sanhaja union.
Yahya determined on a forced march back north to recover the city, but the Godala suddenly decided to call it quits, and broke away from the Almoravid coalition.
Under Abu Bakr's leadership, the Almoravids would re-conquer Sijilmassa, and go on to a spectacular career, conquering most of the rest of Morocco in the 1070s and eventually come back down to finish off what remained of Ghana in the 1080s.
Around 1057, not long after recovering the city, Abu Bakr appointed his nephew Ali ibn Yahya as the Almoravid governor of the Sijilmassa, a post he seems to have held down to 1069.