One of the objections and questions put forward by some people regarding the event of Karbala is why Imam al-Husayn (as) gave his companions permission to go and leave him alone at Karbala on the night of ‘Ashura.
Imam al-Husayn (as) knew that he was facing a large number of enemy soldiers whose hearts did not possess any mercy at all. He knew that fighting with them was certain. He also knew that in any war, there is need for help from friends and supporters.
Why then did he give his companions permission to leave the desert of Karbala? Why did he advise them to take advantage of the darkness of the night and leave him alone?
Two types of permission from Imam al-Husayn (as)
With recourse to history, we come to know that Imam al-Husayn (as) gave two types of permission to his companions.
A. General permission
Imam al-Husayn (as) addressed his companions in a general way,

“And after this; verily, I have never known companions more loyal and better than my companions. I have never found household members more excellent in observation of bonds of relationship than my Household (the Ahl al-Bayt). Allah will reward you graciously on my behalf. Beware! I strongly predict that the day of our confrontation with them will be tomorrow. Beware! I have given you permission to leave. You are all free to go. I do not and will not reproach anyone of you for doing so. This is a night whose darkness has covered you like a mounted camel. Therefore, take advantage of it and make your escapes.”1
B. Special or personal permission
History narrates that Imam al-Husayn (as) did not content himself with the general permission he had given to his companions. He went further and gave personal and special permission to some people for certain reasons.
Imam al-Husayn (as) addressed Muhammad ibn Bushr Hadrami on the night of ‘Ashura in this way, “Your child is being held captive in Marzari.” Muhammad ibn Bushr replied, “I entrust his captivity and mine to Allah and His account. I would rather die than live to see my child in captivity. I would not desire to remain alive after him.”
When Imam al-Husayn (as) heard this, he said, “May Allah have mercy on you. You are free from any obligation to me. Go and strive hard to free your child. You are not obliged to me at all.” He answered, May the wild beasts tear me to pieces alive if I should leave you.” Imam al-Husayn (as) gave him some clothes and said, “Give these clothes to your son so that he may exert himself to free his brother.” The value of those clothes was one thousand dinars.2
Imam al-Husayn (as) came out of the tents in the heart of the night to examine the pits and holes which were in the desert. Nafi‘ ibn Bilal followed the Holy Imam (as). Imam al-Husayn (as) asked him, “Where are you going?” Nafi‘ ibn Bilal answered, “O son of Allah’s Prophet! Your movement towards this rebellious army has aroused my fears.”
The Holy Imam (as) said, “I came out of my tent in order to examine the highs and lows of this desert so as to be able to distinguish the enemy’s military strategic position.” Imam al-Husayn (as) took Nafi‘ ibn Bilal’s hand into his hands and said, “Verily, it is exactly as I have already predicted. I swear upon Allah!
The promise cannot be abrogated.” After this he said, “Are you not going to take advantage of the darkness and pass through these two hills in the heart of the night in order to save yourself?”Nafi‘ ibn Bilal fell at Imam al-Husayn’s (as) feet and started kissing them. He said, “May my mother mourn for me! My sword is worth a thousand dinars and my horse is worth a hundred dinars. I swear upon Allah who holds me indebted for the favor of your presence! I will never leave you.”3
The reality of permission and consent
The question which is put forward at this juncture is this: Does this permission mean that Imam al-Husayn (as) gave his companions clearance from duty and acquittal from the obligation of fighting? Does it imply that they were free from any obligation if they left him alone in those dangerous circumstances? Does it denote that no sin and requital would be recorded for them if they left their Imam unaided? Or is there a deep secret lying beneath this permission?
We believe that Imam al-Husayn (as) had a number of aims in mind when he gave his permission:
1. When Imam al-Husayn (as) made his sermon of giving permission, he wanted to allow the ones who still harboured doubts about his uprising to leave and not get involved in a war they did not fully believe in. The Holy Imam (as) wanted to let those who had joined him for the sake of worldly position, power and money quit the battleground.
2. Additionally, the presence of people lacking strong motivation and character can harm an army and is not beneficial. This is because such people spread the fear and uncertainty which they feel in their hearts to the other soldiers in the camp. This reduces the morale and discipline of the entire army. Fear is contagious and can cause irreparable division among the soldiers of an army.
3. We can also infer that Imam al-Husayn (as) was inspiring his true companions with this kind of discourse to strengthen their determination for war. He was stimulating his true and loyal companions to be even more steadfast in his defence.
4. We can deduce that because the tents were pitched close to each other and the people could hear each other’s voices, Imam al-Husayn (as) wanted his family members to hear for themselves the answers that would be provided by his loyal and faithful companions. This would warm their hearts and raise their spirits high.
5. We can infer that with his speech Imam al-Husayn (as) intended to motivate his followers for a holy war and free them from tribal and clan bigotry. He wanted them to defend him not because of tribal ties, but out of divine motivation to defend what is right and true. He wanted to motivate them to come to the defence of Islam, the faith and monotheism.
Imam al-Husayn’s (as) aim when he made his speech was not to acquit his companions from duty and obligation. In these sensitive circumstances, no one had any pretext for leaving his Imam alone when he needed their help the most.
In reality, the battle that Imam al-Husayn (as) was involved in was such that there was no need for him to ask for help from his companions. Everyone was duty-bound to aid the Holy Imam (as). The very fact that Imam al-Husayn (as) found himself in these circumstances is a call of invitation for help. Is it not incumbent upon people to protect the life of Allah’s trust on earth?
6. When Imam al-Husayn (as) witnessed that his companions possessed sincere and honest intentions, he prayed for them and said, “Lift your heads to the sky and see for yourselves.” When they looked up, they observed their places in heaven. We can deduce from this that Imam al-Husayn (as) wanted to get their allegiance and loyalty before showing them their places in heaven. Seeing and witnessing their places in heaven would strengthen their wills and make them fight with more determination and certainty.
7. On the day of ‘Ashura, Imam al-Husayn (as) was repeatedly asking for help. He would say,

This is not compatible with giving his companions permission to leave and setting them free from any obligation.
8. It has been narrated in some history books that Imam al-Husayn (as) entered Zaynab’s tent after talking to Nafi‘ ibn Bilal and the other companions. Nafi‘ ibn Bilal stood outside Zaynab’s tent waiting for Imam al-Husayn  (as)to come out. He heard Zaynab’s voice. She was talking to Imam al-Husayn (as).
She was saying, “Have you tested the determination and will of your companions? I fear that they will leave you alone and surrender you to the enemy when the war gets fierce and the enemies intensify their attack on you.” Imam al-Husayn (as) said, “I swear upon Allah! I have tested them. I have observed that they are capable of being steadfast and patient during hard times. They are more attracted to death than a newborn is attracted to its mother’s breast for milk.”4
Opposing positions
In order to substantiate that the permission granted by Imam al-Husayn (as) meant that everyone was at liberty to quit the scene of fighting and was exempted from obligation on the condition that they should go far enough that the voice of Imam al-Husayn (as) calling for help and assistance should not reach them, the sceptics have resorted to certain incidents which came to pass.
One of these incidents was when Imam al-Husayn (as) encountered ‘Ubayd Allah ibn Hurr Ju‘fi in the palace of Bani Maqatil. Imam al-Husayn (as) invited ‘Ubayd Allah ibn Hurr Ju‘fi to join him and become one of his helpers but ‘Ubayd Allah ibn Hurr refused and withheld his help from the Holy Imam (as).
Imam al-Husayn (as) said, “I advise you then to do all that is in your capacity to avoid hearing us, the oppressed, when we call out for help. I recommend that you do all that you can to avoid witnessing what will befall us. Because I swear upon Allah! No one who hears our oppressed voices and withholds his help from us will be exempted from the fire of hell."
Response
If Imam al-Husayn (as) had set everyone at liberty to leave him, then why did he ask for help and assistance from ‘Ubayd Allah ibn Hurr Ju‘fi? Yes, it is true that the Holy Imam (as) asked him to go so far that he could not hear the Imam’s voice calling for help against the oppressors or witness the killing.
The Imam was trying to help him not to get caught up in an even greater sin than refusing his help because  on the Day of Judgement, any person who hears an oppressed person calling for help and withholds his help  will meet with the wrath of Allah. If that oppressed person happens to be a Holy Imam, the person who  withholds his help will certainly be afflicted with harder retribution than the one who does not hear or witness the battle.
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References
    1. Muqarram, Maqtal al-Husayn (as), p. 212.
    2. Ibid.
    3. Ibid.
    4. Ibid., p. 219.