I don't understand: why would the Mongols feel slighted by the Arabs? The Arabs had done nothing to warrant the Mongols' wrath aside from killing the odd messenger, especially to the extent of the damages caused by the Mongols. The Mongols did not have a bitter history with the Arabs, which is why the Arab conquest of Persia is incomparable to the Mongol conquest of Arabia, both in terms of war tactics and motivation.
The Mongols invaded the Persianate Khwarezmites by surprise after Genghis Khan himself had written a letter to their emperor saying "you rule the rising sun and I the setting sun", implying they were equals. Not only did the Mongols invade them by surprise without warrant, but they savagely destroyed cities and massacred whole populations. They were essentially destroyers of civilization. This is but one example of many examples as to how the Mongol invasions were oftentimes by surprise and without historical warrant. They were little more than barbarians who destroyed civilizations and stole their wealth. The fact that they were indeed barbarians is apparent when we see how quickly their empire faded, as they did not have the cultural and political sophistication or intelligence to administer diverse individuals and keep their own forces united.
The Arabs, on the other hand, conquered Persia and put a temporary end to centuries of conflict and rivalry which was oftentimes initiated by the Persians. They had legitimate motivation - one rooted in history - to conquer the Persians. When they finally accomplished their task, they did not destroy the rich Persian culture or forcibly tear down its contributions to civilization. They instead maintained many of the political and economic structures already in place in return for tax. What is interesting is that conquering the entire Persian empire was not the original plan of the Rashidun Arabs, who believed their army would be stretched far too thin in pursuing such a task. Caliph Umar was recorded as wishing for a "wall of fire" to separate the Arabs and Persians, and did not want to send his troops across the Zagros mountains. Strong Persian counterattacks forced Umar to push ahead.
In terms of moral justification, there is little to compare between the Arab conquest of Persia and the Mongol conquest of Arabia. The Mongol conquest of Arabia did not have a historical prelude; similarly, the Mongol conquest of the Khwarezmites involved surprise tactics and deception and lacked discernible historical justification. Both of these invasions displayed brutal military tactics which also saw the deliberate destruction of entire cities. The Arab conquest of Persia occurred during an era in which Persian ruling over traditional Arab lands was at its zenith; after the dust had settled, Persian political and economic structures were left largely unchanged, their religion and culture treated with surprising tolerance, and most importantly, their people unharmed and their civilization maintained.
I don't consider any of these conquests to be distinctly "moral" or "immoral", but it's fairly simple to see which ones were "more moral" than others. Hopefully this answers your question.
I'm kind of running around, but, thank you for your answers. Let me ask your opinion on a couple of others. I might get back late, but, I am enjoying the topic.
(1) Do you think that the Mongolian invasion of China was warranted?
(Note that the Yuan Dynasty was successful and China flourished under the Mongolian Dynasty. As one historian put it:
A rich and culturally diverse society developed in China during the Yuan Dynasty)
(2) I was under the impression that assassination and intrigue plagued the Arab Empire soon after it was founded?
(3) What is your opinion of the Arab wars in Spain?
Thanks
M
PS: This is just one big side note, and I don't want to detract from the above 3 questions. Even though from your point of view the Mongolian invasion was unwarranted and the Arab one was, when I asked two friends (both named Reza and ironically both are from Tehran), this exact same question (To compare the two). They both (at different times, these guys never met one another) said the same sort of thing.
Both said the Mongolians came and conquered them, but, the Mongolian didn't change their culture. That's important to them - to respect their "Persian" culture.
The Arabs on the other hand, did everything in their power to eradicate what it means to be "Persian". Their religion was changed. Their language was changed. Every aspect of their life was changed. And because of this they both said they feel the Arabs "raped" their Persian culture.
Now, this got me to thinking of a modern example. And I got to thinking of Modern day Tibet.
So, with one last question (#4). The Chinese have, over 1000s of years, controlled to some level Tibet. In the 1950s they solidified their control (mainly over worries the British were going to set up base). Tibet is a mass farm land for China. They are working hard to improve the lives of Tibetans. And they have too. YET, the Tibetans don't seem to agree? Funny that?
Do you feel China had the moral right to conquer and control Tibet?