The prestigious Spanish military journal “Historia de la Guerra” has published an article by Kaveh Farrokh and Javier Sánchez-Gracia on the Arab invasion of the Sassanian Empire in the 7th century CE (click the link below in for downloading the entire article):

Farrokh, K., & Sánchez-Gracia, J. (2021). La invasion Arabe de Persia [The Arab invasion of Persia]. Historia de la Guerra, 19, pp.5-12.

This article outlines the Arab invasion of the Sassanian Empire by first examining the devastating military consequences of the war with the Romano-Byzantine Empire (610-628 CE) in which each of the antagonists are estimated to each have lost at least half a million of their top professional officers and combatants. The severe military weaknesses as a result of that war provided opportunities for the rising Arabian caliphate to embark on the conquest of Persia.
Omar Ibn Khattab reviewing his troops during the invasion of the Sassanian Empire (Source: Historia de la Guerra).
The Arabian invaders, notably under the brilliant genius leadership of Omar Ibn Khattab, were able to strike at just the critical moment in history when the Sassanian and Romano-Byzantine Empires had been militarily exhausted as a result of their nearly two-decade war.
A historical reconstruction by Tim Newark (illustrator Angus McBride) of the Arab invaders entering into the Mesopotamian interior of the Sassanian Empire – the caption written by Tim Newark for the above illustrations reads as follows: “Muslim Arab warrior and his Ethioipian slave tackle a Sasanid Persian clibanarium, Mesopotamian desert, 7th century” (Source: Newark, T. (1988). The Barbarians: Warriors and Wars of the Dark Ages. London: Blandford Press).
The remainder of the article outlines the details of the battles, chronology and geography of the Arabian conquests of the Sassanian Empire in the 7th century CE.
A chronological map by Kaveh Farrokh outlining the stages of the Arab conquests of the Sassanian Empire in the 7th century BCE (Source: Farrokh, page 267, Shadows in the Desert: Ancient Persia at War-Персы: Армия великих царей-سایه‌های صحرا).  Northern Iran (notably Tabaristan and the Dailamites) however successfully repelled several Arab attempts at conquest with Sassanian enclaves reputedly still in place into the later Islamic eras; note also the Arabian caliphate’s western expansion into the Romano-Byzantine realms.