Freedom Of Mind
As a Persian I could assure you that it was spread by sword. The very beginning of Islam as a major religion was by power of swords, because Muhammad, unlike many other prophets, did not believe in secular governments and he always told people that as the prophet he must also be the ruler of Hijaz, today known as Saudi Arabia.
When he gathered enough followers after some years, he attacked the tribes that ware ruling Mecca and Medinah and became the king, or in his words, Khalifa which means God's substitute.
So if it wasn't for swords, there would be no Islam.
In my opinion becoming an emperor was his main goal, since he took people that were overlooked in society, the ones that were outcast and had no power and formed an army, and promised most of them power and wealth, probably his most infamous promise is the one he made to Ali to make him his successor, but it was overruled when Mohammad died and a result, now we have Shia and Sunni.
So why Islam is spreading, is it because of the sword.
That is silly,you can never make some one believe by force and even if you can then for a limited time.
Muslims are approaching 2 billions, where is the sword ?
Do the Persians nowadays go to pray in the mosques at dawn because of the sword ?
Your rationalizations is irrational and looks so stupid to me.