It is easy to give a straightforward account of the reasons that caused the downfall of the Tokugawa shogunate. To sum up, it was both "internal disorder and external threat" that brought down the Tokugawas. My intention, however, is to present the answer in a more thought-provoking approach.

Internal disorder -- the first part of the answer is to give an account of those social, economic, financial, intellectual, and political changes that undermined the foundations of Tokugawa rule. (for details, see previous sections) As a result of these factors, we can say that "By the early 19th century the facade of the shogunate, firm though it seemed, hid from view a very creaking governmental structure that was ready to collapse if it were subjected to a really challenging blow." (Richard Storry)
To this extent, we can agree with one school of historians that the shogunate would have collapsed (sooner or later) in view of these fundamental, destructive forces within the regime, even without the forced opening of Japan. The key factor, nevertheless, is that of "time". (sooner or later) This leads to a further question: Why did the shogunate fall by 1868 (15 years after the coming of Perry)?

External threat -- the forced opening of Japan by the West since 1853 doubtless contributed to the collapse of the Tokugawas. Since the bakufu fell so rapidly (within 15 years) after Perry's arrival, it is easy to conclude that the coming of the West provided an impetus that accelerated a collapse that was inevitable. The interesting question is: Why was such external threat an impetus?
The answer has two aspects: firstly, the coming of the West in itself brought many harmful consequences to the shogunate. For instance, it exposed the weaknesses of the bakufu. Secondly, it intensified and aggravated those internal changes of Tokugawa Japan. It contributed to and was itself one of the threefold challenge that brought down the Tokugawas between 185 3-68. (for details, see previous sections)

Threefold challenge -- it is much better to explain that between 1853-68, the bakufu was facing a threefold challenge of which the western intrusion was only one. The other two came from the tozama daimyo and the Japanese emperors. This combination was too strong for the already weakened shogunate to overcome. The inevitable took place in 1868 -- the Meiji Restoration replaced the Tokugawa Shogunate.

Conclusion -- First, long before Perry arrived, the Tokugawa regime had been under indirect attack from many quarters inside Japan. The slow but irresistable pressure was eroding the foundations of the bakufu. In this connection, the Tokugawa shogunate would have collapsed even without the coming of the West; but the process might take decades, to say the least. Second, the forced opening of Japan accelerated the whole process, and it took only 15 years for the Tokugawas to step down after 1853. This is sufficient to prove that the western intrusion was a mighty accelerator in bringing about the downfall of the bakufu. Third, between these 15 years (1853-68), the Tokugawa bakufu was confronted with a threefold challenge in the face of which it collapsed. Within these years, these conflicting forces shaped the history of Japan and manifested itself m the sonno-joi movement. Shogun Keiki realized that he was fighting a dying cause, and therefore he put forth no serious resistance and voluntarily surrendered his powers to Meiji Emperor in November 1867. A three day battle in January 1868 sealed the fate of the Tokugawa supporters. The Meiji Restoration as such was largely a bloodless incident.