In your answers you said: "if the leaders or rulers would have treated their land with fairness, then their rule would have lasted for long time". Are you talking about the four rightly guided Caliphs? What we have seen in history is that these four rightly guided Caliphes had set such a wonderful example that even the Westerns are forced to admit their administrative qualities and achievments. You know about Hazrat Umar (ra)'s administrative qualities and achievements. Yes nobody is perfect. Can you please provide me with some references from the history to show how Hazrat Umar (ra) actually treated non-Muslims under his rule or leadership?
Actually I made it clear in my reply that I did not meant the 4 rightly guided Khulafa. I wrote:
"We know that the first Khulafa of Islam (who were also the closed companions of the Prophet) did all they could to treat the conquered lands with fairness. However this does not seem to be the case for every Khalifa that came later."
As for references for the justice of Umar (ra) with non-Muslims, I thought it might be more helpful if I provide you with an online article that is in a good detail. Unfortunately it has not provided references but I can say that it is written using the scholarly references:
Please find it here:
The section that discusses the justice of Umar (ra) to non-Muslims is in the chapter titled "The Dhimmis".