Hadrat Khalid bin Walid (R.A.) proved to be the most successful general of Islam. With a handful of troops he was able not only to overcome all internal rebellions but also to make Arabia safe for Islam. Then he proceeded to Iraq and gained victory after victory. It was for his abilities, which were foreseen by the Holy Prophet (Sallallahu 'alaihi wa Sallam), that Khalid was given the title of "Saifullah" i.e. Allah's Sword.
As-Siddiq al-Akbar was quick to recognise Khalid's (R.A.) ability he put him incharge of the Iraqi campaign. Hadrat Khalid's performance in this campaign has no parallel in history. With an army of a little more than ten thousand men, Khalid (R.A.) overcome a major part of Iraq and brought it under the banner of Islam. At many times he defeated enemies twenty times larger than his own army. Hadrat Khalid (R.A.) knew that success did not lie in the hands of majority or minority, it lies in the hands of Allah Almighty. He declared many times that they were not fighting for the sake of land or fame, their aim was to proclaim Allah's Kalimah (Allah's message). When Allah was with them they did not fear any power. Once he wrote a letter to a commander of Persian army near Hira
"All praise is due to Allah who humbled your pride, and dis-united you, and destroyed your might. Accept Islam you will be in peace; or pay Jizya (the Defence Tax) and let us spread Allah's Message. If you do not accept any of these conditions then listen I have brought with me such people who love death more than you love life".
In Iraq Hadrat Khalid fought fifteen battles and won complete victory in all of them.
Hadrat Khalid was not only a great general and conqueror but also a great administrator. He appointed a deputy and a Qadi (Judge) at every place which he conquered. Some Muslim teachers were also left to teach Islam to the converts and to preach to non-Muslims by peaceful means. Seeing the character of these noble souls and the natural beauty of Islam, most of the time, entire population entered the folds of Islam. It is a fact that by sword or force only tongues could be won not the hearts. It was really the character of Muslims in those days that won the hearts of entire population where they lived. This is a clear proof that Islam never spread by sword or force. Sword was used to clear the way for preaching this truthful way of life, and when people realised its value, grandeur and significance they went all out for it. Iman (Faith) is the affirmation of the heart not of the tongue, and the heart could never be won by sword. Character has greater force than sword and this force was, of course, with the Muslims.
Unlike most of warriors Hadrat Khalid, the Saifullah (R.A.), was an extremely kind-hearted man. He had given strict orders to his army to follow all the principles of Islam. They were not allowed to do any harm to farmers and other civilians. The army treated the civilians with kindness and respect. This was a surprise for the conquered people. Instead of molesting the women they found the soldiers worshipping and crying before Allah all night. This was something new for them. This was something new for them. It was said about Muslim conquerors of the time: "They rode on the back of horse during the day (to fight in the Path of Allah) and on the back of Mussalla (the piece of cloth used for prayers) in the night (to worship Allah). "The word by word translation is given here to put emphasis, it actually meant that they fought in the path of Allah throughout the day and worshipped Allah throughout the night. Of course even during the day they were punctual in obligatory Salats.
As mentioned above the month of Dhul-Qa'dah, 12 A.H. marked the end of Hadrat Khalid's campaign in Iraq. He then returned to the headquarters at Hira. After resting for a while he took a few men with him and came to Mecca for Hajj and returned to Hira after performing the Hajj. On hearing about Khalid's presence at the Hajj, Hadrat Abu Bakr was amazed but he forbade him not to leave his army alone in future.
Copied from 'THE ISLAMIC SCHOLAR'