The westernmost territory of islam's expansion was

The westernmost territory that the Muslims have gained during its so called holy war was the Iberian peninsula, which is the present-day Spain and Portugal. This was achieved during the period of Early Caliphs and by the later Umayyads in 610 to 750 CE. However much of its westernmost territory due to the continuous Christian reconquest done on the peninsula. The last territory fell on the 1400s and most Muslims were expelled from the peninsula if they refuse to change their religion to Christianity. They may have been expelled but their culture lived in the Cities especially in Alhambra and Cordoba.  

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