Islam was spread by sword alone.
Satbir Singh Bedi | 19 Mar 2015

It is considered that Islam was spread by sword because of the following militant verses in the Quran, which were, however, contextual in nature:

"8.12.(Remember) when your Lord inspired the angels, "Verily, I am with you, so keep firm those who have believed. I will cast terror into the hearts of those who have disbelieved, so strike them over the necks, and smite over all their fingers and toes." 
9.5.Then when the Sacred Months (the Ist, 7th, 11th, and 12th months of the Islamic calendar) have passed, then kill the Mushrikun (see V.2:105) wherever you find them, and capture them and besiege them, and prepare for them each and every ambush. But if they repent and perform As-Salat (Iqamat-as-Salat), and give Zakat, then leave their way free. Verily, Allah is Oft-Forgiving, Most Merciful.
9.29. Fight against those who (1) believe not in Allah, (2) nor in the Last Day, (3) nor forbid that which has been forbidden by Allah and His Messenger (4) and those who acknowledge not the religion of truth (i.e. Islam) among the people of the Scripture (Jews and Christians), until they pay the Jizyah with willing submission, and feel themselves subdued.
49.9. And if two parties or groups among the believers fall to fighting, then make peace between them both, but if one of them rebels against the other, then fight you (all) against the one that which rebels till it complies with the Command of Allah; then if it complies, then make reconciliation between them justly, and be equitable. Verily! Allah loves those who are equitable. 
5.54. O you who believe! Whoever from among you turns back from his religion (Islam), Allah will bring a people whom He will love and they will love Him; humble towards the believers, stern towards the disbelievers, fighting in the Way of Allah, and never afraid of the blame of the blamers. That is the Grace of Allah which He bestows on whom He wills. And Allah is All-Sufficient for His creatures' needs, All-Knower. 57.25. Indeed We have sent Our Messengers with clear proofs, and revealed with them the Scripture and the Balance (justice) that mankind may keep up justice. And We brought forth iron wherein is mighty power (in matters of war), as well as many benefits for mankind, that Allah may test who it is that will help Him (His religion), and His Messengers in the unseen. Verily, Allah is All-Strong, All-Mighty."
However, these people who converted Islam by sword forgot the following quranic injunctions which asks to convert disbelievers by tact and wisdom as well as beautiful preaching:
"6:108. Revile not ye those whom they call upon besides Allah, lest they out of spite revile Allah in their ignorance. Thus have We made alluring to each people its own doings. In the end will they return to their Lord, and We shall then tell them the truth of all that they did.
16:125. Invite (all) to the Way of thy Lord with wisdom and beautiful preaching; and argue with them in ways that are best and most gracious: for thy Lord knoweth best, who have strayed from His Path, and who receive guidance.
29: 46. And dispute ye not with the People of the Book, except with means better (than mere disputation), unless it be with those of them who inflict wrong (and injury): but say, "We believe in the revelation which has come down to us and in that which came down to you; Our Allah and your Allah is one; and it is to Him we bow (in Islam)."
So till the time anti-conversion law is passed we may bring the latter injunctions to the notice of Preacher like Zakir Naik.