What caused the unity of japanese shogun to fall apart?

The main driver of the downfall of the shogunate was the exposure to outside countries, specifically their weapons systems. The shogunate had made very limited use of firearms. However, outsiders, such as Admiral Perry, made clear just how far the Japanese were behind in terms of their military capabilities. This drove the overthrow of the shogunate and the beginning of the Meiji period of modernization.

The answer is challenging and taxing because arduous means when something is hard and tiring and the excerpt provides clear detail why that's the answer