What role did the church union play in the history of Byzantium?

It was assumed that the church union would provide an opportunity to repel the invasion of the Ottoman Empire. By 1453, the Turks had already captured most of the former Byzantine lands in Asia and Europe. And they tried to capture the city itself, which was literally isolated by Turkish lands. The Church Union was supposed to unite Catholicism and Orthodoxy under the unified patronage of the Pope. However, it did not happen – the parties did not manage to agree. In addition, the Catholic rulers were in no hurry to go to help and fight in the far east.