Full answer (delay in appearance)
The successful execution of a revolutionary order depends on certain pre-requisites and on the existence of a particular atmosphere. Unless these conditions are fulfilled and that atmosphere is created, it cannot achieve its objectives. As regards the Divinely ordained shape of things it has two aspects. As far as its missionary aspect is concerned it, being Divinely ordained, does not depend on any congenial atmosphere, but, as far as its operational side is concerned, its timing and success are linked with conducive circumstances. The same was the reason why the pre-Islamic period of five centuries had to elapse before the Last Divine Message came to Prophet Muhammad, though the world had been in dire need of it since a far earlier time. It was delayed only because its successful completion was linked with certain suitable circumstances.
The conducive circumstances, which affect the accomplishment of the change, include those which create a suitable atmosphere for it and those which determine the right moment for the beginning of the operation.
For example, the revolution which was successfully led by Lenin in Russia was linked to certain factors, such as the outbreak of the First World War and the decline of the Czarist regime. In addition, there were some minor factors also. For instance, Lenin's safe journey during which he secretly slipped into Russia. If he had met with some accident that might have impeded his entry into that country at that time, the revolution would possibly have been delayed.
It has been the unalterable practice, decreed by Allah, that the actual implementation of a Divine revolution is linked up with such objective circumstances as creating the right climate and general atmosphere for its success.
That is why there was a long gap of several hundred years before the appearance of Islam during which period no prophet was raised.
No doubt Allah is All-powerful. He can miraculously remove in advance all difficulties and obstacles impeding a Divine Mission. But He does not do so, because the tests, trials, and tribulations through which man gains perfection require that a Divine revolution should come about in a natural and normal manner. This does not mean that occasionally Allah does not intervene in arranging certain details not related to the creation of the right atmosphere but which tend to give an impetus to the revolution. The Divine help rendered by Allah to his friends at critical junctures for the purpose of protecting their mission was of this nature.
The fire set alight by Nimrud did no harm to Abraham; the hand of the treacherous Jew who had drawn his sword to kill Muhammad was paralyzed and, a strong wind hit the camp of the infidels who had besieged Madina during the Battle of the Ditch and demoralized them.
In all these cases help was rendered at a critical juncture, but only after the right atmosphere for the desired change had already been created in a natural manner.
On this basis, when we study the position of the Imam Mahdi, we find that the revolutionary task which has been entrusted to him like any other process of social change is linked with certain circumstances which will provide the right climate for its success.
Hence, it is natural that it should be timed accordingly. It is known that the great task, for which Imam Mahdi has prepared himself, is not of a limited nature nor is it confined to any particular region. His mission in fact is to revolutionize the world order in its entirety. It is to rescue mankind from the darkness of vice and to usher in an era of light and guidance.
For such a gigantic revolution the mere existence of a task and a leader is not enough, otherwise, it would have been accomplished during the period of the holy Prophet himself. Such a revolution requires a specific climate and a general atmosphere conducive to the fulfillment of all the prerequisites.”
From a man's viewpoint, the frustration and disillusionment of a man of culture may be considered to be the basic factor in creating the right climate.
This feeling stems from the failure of diverse cultural experiments. Only then does a man of a culture feel that he is in need of help and turns to the unknown.
From the material angle, the modern conditions of life may be regarded as more suitable for the fulfillment of a mission on a world level than the conditions which prevailed at the time of occultation, for now, the distances have been shortened, the chances of contact between various people of the world have been improved and better facilities for a central organization to enlighten mankind on the basis of the new message have become available.
It is true, as pointed out in the question, that the military power and the war equipment which the Awaited Leader would have to face have enormously grown, but it is to be remembered that material power is of no consequence when a man is moralized and is determined to fight against injustice.
Many a lofty civilization in history has collapsed at the first touch of an invader because it was already dilapidated and lacked the power of resistance.