Islamic Conquest of India
The armies of Mohammed first left Arabia ~ 620 AD. They reached Sassanid Persia by 633AD. By 650 AD, Persia was conquered. By 800 AD, Zoarastrianism was extinct.
Starting 620 AD, the same time they reached Sassanid Persia, the Arabs took over huge parts of Egypt, Iraq, Syria, and Palestine from the Byzantine empire. This was completed by ~ 650 AD. The Arabs reached into parts of Anatolia but were repulsed by the Byzantines. However, by 1075 AD, almost all of Asia Minor was Islamic.
After this from 650 AD to 700 AD, the Arab armies conquered all of North Africa and converted huge swathes of people into Islam.
In 710 AD, the first Arab expeditionary force reached the Iberian Peninsula. By 725 AD all of Iberia was under Arab control, huge conversions began here also, but the Cordoban caliphate decided to disallow forcible conversions and the process became more organic and slowed to a crawl. As one can see, Arab and Islamic armies were unstoppable.
Now let us look at the East of Saudi Arabia.
By 720 AD all lands from Saudi Arabia right to the Sindh were captured by the Umayyads. Afghanistan though remained under Hindu kings (these wouldn’t be subjugated till around 1,000 AD). From 720 AD to about 1,000 AD and the Ghaznavid invasions India resisted any and all Islamic incursions. THE only culture to do so while every other nation/culture fell like ninepins.
Some claim that Hindus didn’t unite against Islam. The fact is that the two warring empires, The Chalukya’s and Gurjara Pratihara’s set aside their own century of warfare and formed an alliance that then not only decimated Umayyad forces, it reoccupied all of Rajasthan and Gujarat captured by the Arabs and sent them back across the Sindh. From then till 1000 AD, the Arab armies tried many times to retake lost territory but failed.
So, what changed in 1000 AD?
Military power and tactics. By 1000 AD, Islamic conquests of India were now led by Turkic Islamic tribes or Central Asian, that is, Mongolian tribes. These tribes used entirely different tactics, they were the prototype of the Mongols. They had highly mobile Cavalry archer units, and the Turks had superior artillery that they borrowed from the Byzantines and Western Europe. This combination is unbeatable, as we were to see when all of the Europe+China+Arab states failed to check the Mongols.
Power Vacuum - After the Gupta kingdom fell, their place was occupied by the Chalukya’s, Pratihara’s, and Pala’s. After about 400 years, the Pratihara’s fell, the Chalukyas weakened by the Rashtrakuta’s and Chola’s and were on their death bed. It was at this precise point in time Ghazni invaded us.