The verse says:
"O you who have believed, fight those adjacent to you of the disbelievers and let them find in you harshness. And know that Allah is with the righteous." (9:123)
The noble verse is telling Muslims the strategy they should follow, if they are going to fight then they should start with the enemy closer to them. This is not a war declaration this verse is teaching Muslims a war strategy to fight the enemy closer to them then the far away enemy.
This strategy is logical since fighting a closer enemy first is better for various reasons which are:
1- It costs less, since you don't have to transport your soldiers and weapons for a long distance.
2- You have more information. Since a closer enemy is an enemy closer to your eye so you have more access to his territory and hence has a more understanding to his capability.
3- The closer enemy is more dangerous than a far away enemy because it is very likely that he will attack your territory suddenly which is not the case for a far away enemy who has to travel a longer distance to attack you so it is easier to spot him.
So the verse is teaching Muslims a fighting strategy, why is that harmful. Notice here that the general rule for waging war in Islam still applies which is NEVER to initiate war against someone who did not attack or harm you. Meaning when we say war it means against someone who either attacked or conspired against Muslims. Muslims NEVER initiate enmity.
"let them find in you harshness" implies that harshness is not the usual attitude of Muslims. This attitude is shown only when needed like during a war. You can never tell a soldier in a war to be nice or funny, it's a war then he has to be "harsh". This "harshness" is actually the reason why a war can stop when the enemy finds out that you are as strong as he is or even stronger then he can stop the fight.
The last part of the verse shows the boundary of this "harshness". The noble verse says "And know that Allah is with the righteous ". According to sheikh sharaawy, the verse means that the Muslim fighter must always remember that he is fighting for the cause of Allah to defend Islam and Muslims not to show off his power. So the pious Muslim fighter, who God promised to help him during the war, must stick to the Islamic ethics during war. He must not harm the enemy if he surrenders. He must never use excessive force without need...etc. Many restrictions are there to limit the word "harshness", these restrictions are the ones the Muslim fighter MUST stick to or else he will not be granted God's help.
So what exactly is the reason to claim that this verse is a call for violence. Is it wrong to teach your soldiers a war strategy? Is it wrong to tell your soldiers to be "harsh" on the enemy DURING WAR? especially when you have already taught them the ethics of war that are considered the limitations on their "harshness".
I will not here discuss the ethics of war in Islam as they are discussed fully in another chapter but I will give here just a sample as is indicated by the “ten commands” of the first caliph, Abu Bakr:
"Do not act treacherously; do not act disloyally; do not act neglectfully. Do not mutilate; do not kill little children or old men, or women; do not cut off the heads of the palm-trees or burn them; do not cut down the fruit trees; do not slaughter a sheep or a cow or a camel, except for food. You will pass by people who devote their lives in cloisters; leave them and their devotions alone. You will come upon people who bring you platters in which are various sorts of food; if you eat any of it, mention the name of God over it"