If Al-Mahdi (as) is already prepared for his great task, then why must we wait? Do not all the upheavals and tragedies in the world necessitate his immediate re-appearance?

I would like first to quote Baruch de Spinoza who said something which may help to shed some light in resolving the problems of this question:
Peace is not a status of non-war, but rather a virtue, a faculty, a disposition for welfare, trust and justice.207

He has actually analyzed that peace is not a simple and independent phenomenon or an easy task. The great revolution of Al-Mahdi (as) requires many preceding events. The Russian revolution was predicted upon several factors, such as the outbreak of the First World War and the decline of the Czarist regime. More incidental factors were also played apart, such as Lenin being able to make a safe journey from Finland into Russia.

It has been the unalterable practice, decreed by Allah (swt), that the actual implementation of a Divine revolution is linked with those objective circumstances that create the right climate and general atmosphere for its success. This is why, before the coming of Prophet Muhammad (saws), there were several hundred years in which there was no prophet on the Earth.

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, even though Allah may intervene in arranging certain details not related to the creation of the necessary circumstances for the success of that mission. Rather, we see that Allah (swt) gives His Divine Help to His friends at critical junctures of their mission, once their mission has already begun. The fire set alight by Nimrud did not harm to Abraham (as). The hand of the Jewish man who had drawn his sword to kill Muhammad (saws)d was paralyzed and a strong wind hit the camp of the infidels who had besieged Madinah during the Battle of the Trench. In all of 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 Al-Mahdi (as), we find that the revolutionary task which has been entrusted to him is, like any other process of social change, linked with certain circumstances which will provide the right climate for its success. The great task for which Al-Mahdi (as) has prepared himself is not of a limited nature, nor is it confined to any particular region. His mission is to revolutionize the world order in its entirety, and 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 (saws) himself.

The most important factor leading up to this revolution is man's despair in his own ideologies and cultural experiments. It is only then that a man of culture will 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 fulfilment 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 war equipment that the Awaited Leader would have to face have grown enormously, but material power is of no consequence when man is determined to fight against injustice. Many a lofty civilization in history has collapsed at the first touch of an invader, because it had already become dilapidated and lacked the power of resistance.208

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207. www. ordtak.no/Baruch de Spinoza

208. As-Sadr, Sayyid Muhammad Baqir, The Awaited Saviour, pp.56-57, London: Al-Khoi foundation, 1996

Adapted from the book: "The Awaited Saviour; Questions and Answers"