The following question was put to Reb Noson of Breslov:
Seeing that Mashiach (the Messiah) hasn’t yet come despite all the efforts of all the very great Tzaddikim in all the previous generations, how will he come in these few remaining and much weaker generations.
Reb Noson told a parable to explain this paradox:
There was once a city that was very well fortified. It was enclosed by a thick stone wall, thought to be impenetrable. A wise king decided to conquer this fortified city. After inspecting the fortifications, he sent his mightiest soldiers to bring down the wall and attack the city. These soldiers fell. He next sent a second wave of less mighty soldiers, and then a third wave, and so on. Before long, his entire army had been depleted and the wall had not come down. But the king did not give up. Once again he circled the city, inspecting its walls. “How can you expect to capture this city if all your mighty soldiers are gone?” he was asked. The wise king smiled. “If you look closely, you will see that though the soldiers could not breach the wall, they did succeed in cracking it. It is no longer strong and impenetrable. Now, with even the weak and wounded I can bring down the wall.” The king then sent his few remaining and weakest soldiers into battle and conquered the city.
Reb Noson explained that though they did not succeed in bringing the Mashiach, all the great Tzaddikim of the previous generation did succeed in cracking the wall of obstacles which stand in the way. Now, though we are weak and haven’t the strength or the power, if we would but make a concerted effort, we could bring the wall down and bring the Mashiach (Ma’asioi U’Meshalim, p. 36-37).