What has been more responsible for the fall of empire(s)internal or external problems

In terms of some, like Russia, it was the internal and external. In older empires like the Roman, it was internal corruption coupled with the relocated tribes. For something like Britain, it was the external push for a need for self determination world wide. 

The Silk Road began as a way for countries to trade things such as Silk, Horses, Nephrite Jade, Lapis Lazuli, Spinel, etc...