How did it even happen exactly? Were Byzantines and Sassanids

sudden, massive conquests aren't uncommon in history. What made this particular one important is the length of time it lasted

>>2728071
Three words.
Khalid ibn Walid

>>2728071
The Byzantines and Sassanids were fighting a war for centuries. By the end of it both nations were way too exhausted, especially the Sassanids who were on the brink of collapse after their wars. It wasn't that hard for the Arabs to take advantage of their situation.

>>2728071
They weren't incompetent but the centuries long war each empire waged against each other put a huge drain in resources and overall made them more exhausted for the on coming Arabs. Also as well, Khalid who was an exceptional general took advantage of their time of weakness.

I'm not too familiar with the Sassanid campaigns, but the Byzantines mostly lost territory through pseudo-revolts. The coming of the Arabs allowed Egypt to cede from Byzantium, which they'd wanted to do since ever; North Africa was basically an isolated enclave since the western empire fell, and Iberia had fallen forever ago. A few victories by the Arab armies and opportunistic (and probably short-sighted) independence movements just let themselves be conquered.

>>2728071
Arabs clearly weren't as incompetent in warfare back then, maybe they actually gave time to proper strategy and working as a collective rather than for personal gain.

>>2728071
Allahus vult