Rahul Mehta Gandhinagar LsCandidate

Around 500 AD, Afghanistan, Western Pakistan, Central Asian Republics, China, Burma, Thailand, Cambodia, Malaysia, Indonesia had all Buddhism
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Arabs and later Islamists managed to defeat and convert Afghanistan, Western China, Western Pakistan, Central Asia. And from Afghanistan, Islamists spread over into India till Bangladesh.
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And Arab traders converted a small portion of Malaysian population, less than 0.1% into Islam. And this 0.1% grew to some 20%, with organic and peaceful conversion, and then became 100% of Malaysia, Indonesia, Phillipines thru peaceful conversions, and also some violence. The violence in Malaysia, Indonesia was not much, as one can even today see almost all temples and buddist monastaries intact, though abandoned.
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So question is --- why couldnt Arab or Central Asian Muslim rulers or Afghan rulers defeat China?
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And why couldnt Islam from Malaysia spread into Thailand , Burma and Cambodia organically they way it spread in Indonesia?
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BONUS QUESTION --- How come Arabs, Turks etc defeat Chinese in Weatern part of China and convert them to Islam, but not mainland China?