People talk about why the Muslims declined, and many academic studies have been done, going deep into the issue and often coming up with abstruse and far-fetched explanations, and many of them miss the mark or don't go to the root.
Allah said: “Truly, God does not change the condition of a people until they change what is in themselves.” (Quran 13:11)
Thus we see that we need to change our state/condition and that is one of the major reasons why the Muslims are and have been in decline.
Yet the way Muslims treat each other, is far from the Prophetic way, and generally indicates selfishness.
The Qur'an said: “They prefer others above themselves, even though poverty be their portion” (al-Hashr, 59.9)
Prophet (peace be upon him) said: “None of you truly believes (in Allah and in His religion) until he loves for his brother what he loves for himself” [Bukhari and Muslim]
Who can say that they’ve met a SINGLE Muslim (apart from the Awliya) who has implemented the above Hadith? You get many religious and charitable Muslims, but almost none prefer others to themselves, or at least love for them what they love for themselves. Until we change this attitude, we’re not going to be an Ummah that will get anywhere. We’ll go from one nightmare to another, like Syria’s oppressed Muslims not getting helped because other Muslim states care about themselves more.
Where are the likes of the Ansar, who when their own families were starving, and the migrant guests needed food too, the Ansar gave the migrants food instead and sheltered them in their own homes?
Go on, prefer others to yourselves. Be fair to people, but don't demand that they be fair to you.