After Imam al-Mahdi establishes his rule, will there be any oppressors remaining in power?
The characters of the universal rule of Imam al-Mahdi will be the worldwide maintenance of peace, security, justice, and the annihilation of the foundations of corruption, decadence, oppression, and crime. This has been the main purpose of all the prophets and establishing worldwide peace and security has always constituted one of their promises. And so the unpolluted souls of mankind at large, ever since the beginning of human history, have been waiting for the time when those promises would be realized. Now since Allah, His messengers, and their infallible successors do not breach their promises, they would not raise the hopes of people without a good reason. This means that this promise will certainly be realized and that a just government will be established, consisting of the aforementioned characteristics.
This promise is so vital and real that its realization can be considered the rationale behind the creation of humankind; a promise that once realized will lead to the unity of the whole of humankind under the banner of monotheism and, by Allah's Mercy, the uprooting of disbelief, polytheism, and hypocrisy from amongst them. The narrations that express this are numerous.