Jihad (English: ; Arabic: جهاد jihād) is an Arabic word which literally means striving or struggling, especially with a praiseworthy aim. It can have many shades of meaning in an Islamic context, such as struggle against one's evil inclinations, an exertion to convert unbelievers, or efforts toward the moral betterment of society, though it is most frequently associated with war.
Many the battles that were fought during the expansion of the Arab-Muslim empires after the time of the Prophet (S) were the same sort of battles that any other empire or state engages in to look after its political, financial or economic interests, although it is true that, as a side effect, it contributed to the spread of Islam by establishing a ruling class who was Muslim. So, from that angle, there is nothing special about them to make them jihad fi sabil Allah.
However, if someone was fighting in defence or for other selfless reasons, this could be considered jihad, just as it would be considered jihad fi sabil Allah today if I risked my life to defend someone who is under attack.
Perhaps for that reason, there is a dua in al-Sahifah al-Sajjadiyah where Imam Zayn al-Abidin (A) prays for the soldiers on the frontier who are defending the Muslim state against enemies.
Also, even in times of jihad with the Prophet (S), whether or not fighting was counted as jihad was according to intention. For instance, some people might have gone to war for glory or financial gain, and so this is not the same thing as risking their lives or enduring battle solely for the sake of Allah.
Imam Ali (AS) never took part in any matter of the governments of Abu Bakr, Umar and Uthman. He did not attend any Juma or Jama'ah with them (Al-Thahabi in Siyar A'laam al- Nubalaa' 9/284.
Valid Jihad must be led or permitted by the Prophet (SAWA) or his real successors.
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The claim of your non Muslim friend is untrue. He might be influenced by wrong anti Islam propaganda.
Quran is very clear that (No compulsion in religion)(2:256). The Prophet Muhammad (SAWA) never forced any one to become a Muslim inspire of being the head of the state in Madina. Non Muslims lived free in their faith during his government.
We never fight to force people to become Muslims and we believe that I forced faith is invalid. We fight just to defend ourselves, our community and our faith when there an attack on us. Allah says in Quran (And fight in the way of Allah those who fight you but never transgress the limits, surely Allah does not like the transgressors)(2:190).
Muslims ruled over many countries e.g Indian subcontinent, Indonesia, Egypt and African countries but never forced people to b4come Muslims. That is why you find after centuries of Muslim rule, millions of non Muslims are still existing and free in their non Muslim faith. Muslims ruled India for ten centuries and find till today the majority of people there are non Muslims. Those who became Muslims opted themselves by themselves to embrace Islam because of the great teachings of Islam with out any force. Egypt after 1400 years of Muslims rule have till today millions of Copts. Theses are living examples of the freedom of faith in Islam.
The sincere believers will obey the orders of Imam Al-Mahdi (AS) in every matter in their life. He will not order all the believers to fight. His aim is not merely fighting but only against the enemies of human beings who are the tyrants. He will clean the whole earth from tyranny and injustice and fill the whole earth with justice and fairness.
He will never ask people to do any thing which they do not know, so, if you have no experience in fighting, you will not be asked to fight, unless you go through suitable training which makes you able to fight. That also only fighting was required in your area, which is not confirmed in every area. We need to prepare ourselves to obey the orders of our Imam (AS) but we can never know now whether he will order us to fight against the enemies of human beings or to build our societies and serve people.
Imam Al-Mahdi (AS) will not fight but only against those enemies who insist on fighting him. Non Muslims will never be forced to become Muslims. The Prophet Muhammad (SAWA) and Imam Ali (AS) never forced any non Muslim to become a Muslim. Imam Al-Mahdi (AS) will be applying Islamic rules in the same prophetic way.
In the name of Allah
During the 9 years of The Holy Prophet's life after Hijrah - depending on the definition of battle - between 80 to 100 counts of Battle between Muslims and non-muslims are recorded in history.
The Holy Prophet -Peace be upon him and his Ahlul Bayt- was personally present and leading the Army in 27 of these Battles that are called Ghazwah (غزوة). The rest were lead by his appointed generals.
It's worth mentioning that most of these battles were resolved without any/major conflict like the Conquest of Mecca.
Here you can read more details about some of these battles:
If someone attacks you it is permissible to defend yourself.