This region along the Persian Gulf coast of modern Saudi Arabia was part of the neighboring territory of Bahrain at the beginning of the era of Islam. Most of its inhabitants became Muslim during the lifetime of Muhammad.
Bahrain became a center of Shi'ih Islamic belief very early on, starting with a strong core of loyalty to the leadership of the Imam Ali. As a result, Bahrain suffered numerous attacks from the forces of Sunni Caliphs over the next several centuries. Gradually, this region also began to earn a lasting reputation for its achievements in philosophy and religious scholarship.
Ahsa became separated from Bahrain in 1521 C.E., when the area was broken up during its colonial occupation by Portugal. The Portuguese were driven out in 1602, but Bahrain remained confined to the island group currently known by that name.
Ahsa finally ended up within Saudi Arabia. It runs along the Gulf coast, south from the border with Kuwait to just north of the peninsula of Qatar, and is roughly 200 kilometers wide (east to west). Some maps and sources refer to it as "Al Ahsa," or the alternative transliteration "Al Hasa."