1. Imam Hussain (pbuh) opposed an oppressive ruler, whereas Imam Hassan (pbuh) was dealing with the people.
One interpretation, according to Martyr Motahhari, is that when Imam Hussain (pbuh) was martyred, he was both protesting and at the same time encouraging what is right and forbidding what is wrong. He rose up against an oppressive ruler, because the ruler was committing oppression, acting immorally and sinfully, and was malicious. So, Imam Hussain (pbuh), as someone who was protesting and as a believer, felt it to be his religious duty to protest against this ruler. This protest was obligatory, and the culmination of the beauty and splendor of his actions was that he continued this protest until he was innocently killed. Martyred for the cause of Allah, Imam Hussain (pbuh) proved his words to be the truth.
Imam Hassan (pbuh) was not in the same situation at all. Imam Hassan (pbuh) was a caliph against whom a person from his own people had risen up in revolt. If Imam Hassan (pbuh) had confronted his own people who had rebelled against him and had lost his life, he would never have been seen to be the brave person who was protesting, who stood up to what was wrong and was killed. Rather, he would have been regarded to be a ruler who was killed by his own agents. Therefore, you can see that the circumstances were completely different in these two cases.
2. Imam Hassan (pbuh) had to sign a peace, even after 10 years of fighting Mu'awiyah!
The second point is that when considering the situation of his own army, the situation in Kufa and the social and political conditions at the time, Imam Hassan (pbuh) saw that it was better that he end the war immediately, and submit the government to Mu'awiyah. If he hadn’t ended the war at that point and had continued the war for another year, two years, five years, or 10 more years, the result would have been the same. That is, with the method of Imam Hassan, which was the divine method and the method of Imam Ali, as opposed to the methods that Mu'awiyah was using, which included buying people's hearts and consciences with money, and also considering the people’s weak faith and the fatigue of the people of Kufa, if Imam Hassan had fought for another 10 years, the ending would have been the same. He would have been forced to submit to Mu'awiyah, but 10 years later. However, what would have happened to the Islamic nation during these 10 years? Thousands would have been killed, and protests against Imam Hassan (pbuh) would have escalated. So, while he was certain, he knew that the conditions did not promise a better future and there was no point in insisting on a war that would lead to more casualties.
3. During the time of Imam Hassan (pbuh) pledging allegiance with Mu'awiyah was not the issue
Another difference is that Imam Hussain (pbuh) was pressured by Yazid to pledge allegiance. Yazid wrote to the governor of Medina, ordering him to put pressure on Hussain ibn Ali (pbuh) so that he would pledge allegiance. What did pledging allegiance mean? Allegiance meant endorsing the caliphate of a vicious, malicious ruler. Imam Hussain (pbuh) said, "I swear by Allah that I will not put my hand in your hand (pledge allegiance) in humiliation. One such as me will never pledge allegiance to Yazid.” He was pressured for pledging allegiance up until the last hour when Ibn Sa'd and others came to him. They told him to either pledge allegiance or to fight. They wouldn’t stop.
But how about Imam Hassan (pbuh)? Imam Hassan was not asked to pledge allegiance to Mu'awiyah. In the ceasefire or peace agreement that was signed between Imam Hassan and Mu'awiyah, some of the conditions were that Imam Hassan was not required to call Mu'awiyah the Leader of the Faithful (Amir-al-Mu’minin), he would not pledge allegiance to him, and Mu'awiyah did not have the right to appoint a successor. In some narrations, Mu'awiyah was to appoint Imam Hassan as the caliph after him and then Imam Hussain. So, the situation was very different. Well, yes, if Imam Hassan was pressured to pledge allegiance, the situation would have been different, and this is the third difference.
4. Imam Hussain (pbuh) confronted an oppressive, anti-Islamic ruler, but during the time of Imam Hassan (pbuh) Mu'awiyah claimed to act in accordance with Islam
The fourth difference is that during the time of Imam Hussain (pbuh), the situation was exactly as portrayed in a tradition from the Prophet. He had said, “If one sees that a ruler is oppressing, tyrannizing, violating the rights of the people, turning unlawful deeds (haram) into lawful deeds (halal), altering God's orders, and so on, it is obligatory for him to confront this ruler.” Yazid, who was reigning, was a perfect example of all of these. He manifested vice, corruption, oppression, turning wrong into right, and openly fighting Islam. What else should Imam Hussain (pbuh) have waited for? He had to confront him. He had to take action against him, and he did. This was not the case at the time of Imam Hassan (pbuh).
Imam Hassan (pbuh), of course, knew either by way of the knowledge he had as the Imam or by way of the wise conjecture of a great man, that Mu'awiyah would do these things one day. However, this was not the case at the time. Mu'awiyah claimed that he was seeking compensation for the blood of Uthman and that he wanted to act in accordance with God's commands. Imam Hassan wrote in the peace treaty that Mu’awiyah should act in accordance with God's commandments and the Prophet's traditions, and he had accepted that he would comply with these. This was the situation in appearances. It is very different from the situation during the time of Imam Hussain. Therefore, the narration from the Prophet did not apply to the time of Imam Hassan (pbuh). This is the fourth difference.
5. People and history praised Imam Hussain (pbuh), but if Imam Hassan (pbuh) had been killed, many questions would have remained
The fifth difference concerns how people at that time and throughout history would have judged differently because of the different circumstances. Imam Hussain (pbuh), who rose up and was martyred, was praised by the people even at his own time. For over 1,300 years after him, Imam Hussain has been praised by both those who follow him as an Imam and those who did not accept him as an Imam, because the situation led to this result. But if Imam Hassan (pbuh) had fought to his death, many questions would have remained unanswered. As the book elaborates in detail, some people would have said, “Didn’t you want the religion to rule in society? Well, Mu'awiyah said he would make the religion rule and he did. Mu'awiyah claimed that he was willing to comply with the Book of God. Why didn't you accept?” If someone argued that Mu'awiyah would have revealed his true intentions after the martyrdom of Imam Hassan with his wrongdoings, the same people who had protested would have said, “Yes, when Imam Hassan was not there, he would have done such things. You (Imam Hassan) should have kept yourself alive. You should have stayed and prevented him.” Therefore, there would have been places for questioning him.
Imam Khamenei, May 1, 1989