Some of the reasons why Jihad for the sake of Allah (SWT) is prescribed in Islam are as follows:

1. The main goal of Jihad is to make the people worship Allah alone and to bring them forth from servitude to people to servitude to the Lord of people.

Allah says (interpretation of the meaning):

“And fight them until there is no more Fitnah (disbelief and worshiping of others along with Allah) and (all and every kind of) worship is for Allah (Alone). But if they cease, let there be no transgression except against Az-Zaalimoon (the polytheists, and wrong-doers)” [al-Baqarah 2:193]

“And fight them until there is no more Fitnah (disbelief and polytheism, i.e. worshiping others besides Allah), and the religion (worship) will all be for Allah Alone [in the whole of the world]. But if they cease (worshiping others besides Allah), then certainly, Allah is All-Seer of what they do” [al-Anfaal 8:39]

Ibn Jareer (Rahimahullah) said:

“So fight them until there is no more shirk, and none is worshiped except Allah alone with no partner or associate, and trials and calamities, which are disbelief and polytheism, are lifted from the slaves of Allah on earth, and religion is all for Allah alone, and so that obedience and worship will be devoted to Him alone and none else.”

Ibn Katheer (Rahimahullah) said:

“Allah commands us to fight the kuffaar so that there will be no fitnah, i.e., shirk, and the religion will all be for Allah alone, i.e., the religion of Allah will prevail over all other religions."

The Prophet (Sallallahu alaihi wasallam) said:

"I have been commanded (by Allah) to fight the people until they testify that there is no god but Allah and that Muhammad is the Messenger of Allah, and they establish regular prayer and pay zakaah, then if they do that, then they save their lives and property from me except for Islamic laws and then their reckoning will be with Allah." [Narrated by al-Bukhaari (24), Muslim (33)]

And he (Sallallahu alaihi wasallam) said:

“I have been sent just before the Hour with the sword, so that Allah will be worshiped alone with no partner or associate.” [Narrated by Ahmad, 4869; classed as saheeh by al-Albaani in Saheeh al-Jaami’, 2831]

This purpose of Jihad was present in the minds of the Sahaabah during their battles with the enemies of Allah. Al-Bukhaari (2925) narrated that Jubayr ibn Hayyah said: ‘Umar sent people to all the regions to fight the mushrikeen… so ‘Umar recruited us and appointed al-Nu’maan ibn Muqarrin to lead us. When we were in the land of the enemy, the representative of Chosroes came out to us with forty thousand troops. An interpreter stood up and said: “Let one of you speak to me.” Al-Mugheerah said: “Ask whatever you want.” He asked, “Who are you?” He (al-Mugheerah) said: “We are some people from among the Arabs. We used to lead a harsh and miserable life, sucking on animal skins and date stones because of hunger, wearing clothes made of camel and goat hair, worshiping trees and rocks. While we were in this state, the Lord of the heavens and the earth, exalted be His name and glorified be His greatness, sent to us a Prophet from amongst ourselves, whose father and mother we know. Our Prophet, the Messenger of our Lord, (Sallallahu alaihi wasallam), commanded us to fight you until you worship Allah alone or pay the jizyah. Our Prophet (Sallallahu alaihi wasallam) told us the message from our Lord, that whoever among us is killed will go to Paradise to enjoy delights such as no one has ever seen, and whoever among us is left will become your master.”

This is the truth that the Sahaabah and leaders of the Muslims proclaimed in their military campaigns.

Rab’i ibn ‘Aamir said, when Rustam the commander of the Persian armies asked him, “Why have you come?”: “Allah has sent us to bring forth whomsoever He wills from the worship of man to the worship of Allah.

When ‘Uqbah ibn Naafi’ reached Tangiers, he rode his horse into the water until the water was up to its chest, then he said: “O Allah, bear witness that I have done my utmost, and were it not for this sea I would have traveled throughout the land fighting those who disbelieve in You, until none is worshiped except You.”

2. Repelling the aggression of those who attack the Muslims.

The scholars are unanimously agreed that repelling the aggression of those who attack the Muslims is fard ‘ayn (an individual obligation) upon those who are able to do that.

Allah says (interpretation of the meaning):

“And fight in the way of Allah those who fight you, but transgress not the limits. Truly, Allah likes not the transgressors” [al-Baqarah 2:190]

“Will you not fight a people who have violated their oaths (pagans of Makkah) and intended to expel the Messenger while they did attack you first? Do you fear them? Allah has more right that you should fear Him if you are believers” [al-Tawbah 9:13]

3. Removing fitnah (tribulation)

Fitnah is of different types:

(a) That which is caused by the kuffaar who persecute the Muslims or apply pressure to them to make them give up their religion. Allah has commanded the Muslims to fight in jihad in order to save those who are weak and oppressed. Allah says (interpretation of the meaning):

“And what is wrong with you that you fight not in the Cause of Allah, and for those weak, ill-treated and oppressed among men, women, and children, whose cry is: “Our Lord! Rescue us from this town whose people are oppressors; and raise for us from You one who will protect, and raise for us from You one who will help” [al-Nisa’ 4:75]

(b) The fitnah of the kuffaar themselves and their preventing others from hearing and accepting the truth. That is because the kafir systems corrupt the innate nature and reason of people, and make them get used to worshiping and submitting to things other than Allah, getting addicted to alcohol, wallowing in the mire of sexual licence, and losing all characteristics of virtue. Whoever is like that can rarely tell truth from falsehood, good from evil, right from wrong. So jihad is prescribed in order to remove those obstacles that prevent people from hearing and accepting the truth and getting to know it.

4. Protecting the Islamic state from the evil of the kuffaar.

Hence the Prophet (Sallallahu alaihi wasallam) ordered that the leaders of the kuffaar be killed, those who incited the enemies against the Muslims, such as the Jews Ka’b ibn al-Ashraf and Ibn Abi’l-Haqeeq.

Another aspect of Jihad is to protect the borders against the kuffaar. The Prophet (Sallallahu alaihi wasallam) encouraged that as he said: “Guarding the border for one day for the sake of Allah is better than this world and everything in it.” [Al-Bukhaari, 2678]
5. Frightening the kuffaar, humiliating them and putting them to shame.

Allah says (interpretation of the meaning):

“Fight against them so that Allah will punish them by your hands and disgrace them and give you victory over them and heal the breasts of a believing people,
And remove the anger of their (believers’) hearts. Allah accepts the repentance of whom He wills. Allah is All-Knowing, All-Wise” [al-Tawbah 9:14-15]

“And make ready against them all you can of power, including steeds of war (tanks, planes, missiles, artillery) to threaten the enemy of Allah and your enemy” [al-Anfaal 8:60]

Hence it is prescribed to fight in a manner that will strike terror into the heart of the enemy.

6. Exposing the hypocrites.

Allah says (Interpretation of the meaning):

“But when a decisive Soorah (explaining and ordering things) is sent down, and fighting (Jihad — holy fighting in Allah’s Cause) is mentioned (i.e. ordained) therein, you will see those in whose hearts is a disease (of hypocrisy) looking at you with a look of one fainting to death” [Muhammad 47:20]

At times of ease and plenty, the Muslims may be joined by those who seek to make worldly gains, and they do not want to make the word of Allah prevail over the word of kufr. These people may conceal their real nature from many of the Muslims, and the most effective means of exposing them is jihad, because jihad means sacrificing oneself but these hypocrites are only indulging in hypocrisy in order to save themselves.

Exposing the hypocrites was one of the major purposes that Allah wanted the believers to achieve on the day of Uhud.

Allah says (interpretation of the meaning):

“Allah will not leave the believers in the state in which you are now, until He distinguishes the wicked from the good” [Aal ‘Imraan 3:179]

Ibn al-Qayyim (Rahimahullah) said:

This means: Allah will not leave you in the state in which you are, where the believers are indistinguishable from the hypocrites, until the believers are made distinct from the hypocrites, as they were distinguished by the test on the day of Uhud, and Allah will not disclose to you the unseen matters by which the one group is distinguished from the other, for they are distinguished from one another in the knowledge of the unseen that He has, but He wants to distinguish them from one another in a clear and visible manner, so that His unseen knowledge will become known and visible.

7. Purifying the believers of their sins and ridding them thereof.

Allah says (interpretation of the meaning):

“And so are the days (good and not so good), that We give to men by turns, that Allah may test those who believe, and that He may take martyrs from among you. And Allah likes not the Zaalimoon (polytheists and wrongdoers).

And that Allah may test (or purify) the believers (from sins) and destroy the disbelievers.

Do you think that you will enter Paradise before Allah tests those of you who fought (in His Cause) and (also) tests those who are As-Saabiroon (the patient)?”[Aal ‘Imraan 3:140-142]

8. Acquiring booty

The Prophet (Sallallahu alaihi wasallam) said: “I have been sent ahead of the Hour with the sword so that Allah will be worshipped alone, and my provision has been placed in the shade of my spear, and humiliation has been decreed for those who go against my command, and whoever imitates a people is one of them.” [Narrated by Ahmad, 4869; Saheeh al-Jaami’, 2831]

Al-Haafiz (Rahimahullah) said:

“This hadeeth indicates that war booty is permissible for this Ummah, and that the provision of the Prophet (Sallallahu alaihi wasallam) has been placed in war booty and not in other kinds of earnings. Hence some of the scholars said that it is the best kind of earnings.”

Al-Qurtubi (Rahimahullah) said:

“Allah sent provision to His Prophet (Sallallahu alaihi wasallam) by means of his striving and He made it by means of the best kind of striving which is earning it by means of force and strength. The Prophet (Sallallahu alaihi wasallam) went out for the battle of Badr to meet the caravan of Abu Sufyaan.”

Al-Qurtubi (Rahimahullah) said:

“The fact that the Prophet (Sallallahu alaihi wasallam) went out to meet the caravan indicates that it is permissible to take booty because it is a halaal source of income. This refutes Maalik’s view that this is makrooh, when he said that this was fighting for worldly gains.

Al-Shawkaani said: Ibn Abi Jamrah said: The scholars of hadeeth are of the view that if the primary motive is to make the word of Allah supreme, it does not matter what else is also achieved.

9. Taking martyrs.

Allah says (interpretation of the meaning):
“If a wound (and killing) has touched you, be sure a similar wound (and killing) has touched the others. And so are the days (good and not so good), that We give to men by turns, that Allah may test those who believe, and that He may take martyrs from among you. And Allah likes not the Zaalimoon (polytheists and wrongdoers).

And that Allah may test (or purify) the believers (from sins) and destroy the disbelievers” [Aal ‘Imraan 3:140-141]

Martyrdom is one of the highest statuses before Allah, and the martyrs are the closest of His slaves to Him. There is no status higher than that of siddeeq apart from martyrdom. Allah loves to take martyrs from among His slaves, who shed their blood for His love and to earn His pleasure, preferring His pleasure and His love above themselves. There is no way to attain this status except by circumstances that may lead to it such as enemies coming against the Muslims.

This was said by Ibn al-Qayyim in Zaad al-Ma’aad.
This is the great wisdom and those who try to put the Muslims off jihad and make them fear it, and say that jihad is no more than death, and making women widows and children orphans, pale into insignificance.

10. Ridding the world of corruption.

Allah says (interpretation of the meaning):

“For had it not been that Allah checks one set of people by means of another, monasteries, churches, synagogues, and mosques, wherein the Name of Allah is mentioned much would surely, have been pulled down. Verily, Allah will help those who help His (Cause). Truly, Allah is All-Strong, All-Mighty”[al-Hajj 22:40]

“And if Allah did not check one set of people by means of another, the earth would indeed be full of mischief. But Allah is full of bounty to the ‘Aalameen (mankind, jinn and all that exists)” [al-Baqarah 2:251]

Muqaatil (Rahimahullah) said:

“Were it not that Allah checked the mushriks by means of the Muslims, the mushriks would have overrun the earth and killed the Muslims and destroyed the mosques.”

Sheikh al-Islam Ibn Taymiyah (Rahimahullah) said in al-Jawaab al-Saheeh, 2/216:

So the kuffar are repelled by means of the Muslims and the worse of the two parties is repelled by the better, just as the Magians (Persians) were repelled by the Christian Byzantines, then the Christians were repelled by the believers of the Ummah of Muhammad.

Al-Sa’di (Rahimahullah) said:

"The world would be corrupted if the kuffaar and evildoers were to prevail."

These are some of the reasons why Jihad is prescribed. We ask Allah (SWT) to bring the Muslims back to their religion. May Allah (SWT) send blessings and peace upon our Prophet Muhammad.