Well Mr Khajur I'd hate to particiapate in this dicussion but while you are derailling this thread by posting your Hindu turned Buddhist professors writing who is trying to prove that Islam spread by Invasions,I had to answer that.
It is rathers stupid and ridiculous to think that Islam spread by force by this recipe there would have been no minorities in Muslim countries because they can use force to convert them,and another question is that if we generalise this for every religon because your writers are assuming that use of power can force the person to change his faith then there would'nt be any Muslims in India because hindu extremists could easily force Muslims to convert to Hinduism as they made various failed attempts in past to do that one example is "The Shud-dhi Movement" which failed badly.
Had sword had power to conquer minds then the whole of India might have been Muslim I mean when you have been for centuries under the rule of Muslims,but as we see practically that its kindness and good behavior that attracts minds not threats,coercion and use of influence and power.
Islam advocates peacefull methods of preaching and spreading religion and strictly forbids any use of force to do this,as clearly stated in holy Quran.
"There is no compulsion in religion, in Islam." (Surah Al-Baqarah Verse:257)
So what were the motives of the Arab Invaders? well certainly not about spreading Islam,but they might be thinking of expanding their empires (because no specific invader has been named so I can not clearly explain this),
So how did Islam spread? well by the Saints and Sufi's and other religious personalities who acted on Prophet Muhammad's(Peace Be Upon Him) Seerat(way of living) and they travelled the whole world preaching it they did that peacefully and nonviolently and because of this even today many non muslim too have respect for them.Corect me if I am worlg but I heared that many non Muslims(Hindus,Budhists,Christian etc) in India also visit shrines of Muslims saints in India.
As for destroying Idols well they destroyed Idols not thier gods Idols and temples can be made again if this could wipe or drive religons out then Hinduism too might well had left India because many Temples and Idols were attack and destroyed by Invaders,(mainly because of wealth like the famous Somnath for instance) dont take these words offensive I am only quoting India because I dont know much about the other areas.
Now where is Mr EjazR? Mr Ejaz will you consider clearing doubts in your countrymens minds regarding the nonviolent spread of Islam throughout the world? or are you still busy spending time on anti Pakistan threads and Pakistan bashing.