It took a long time for the old Roman Empire to collapse. The seeds of its demise blossomed slowly, but the historically astute among the Romans saw the destructive nation-eating plants growing when they first began to appear, but no one would believe them. And when the end came, it came quickly—almost overnight.
The Roman Empire was so rich and prosperous and so militarily mighty that no one within the empire could even imagine that a collapse was possible. I'm sure that what I've heard hundreds of Americans say, "It can't happen here," also echoed throughout the old Roman Empire for generations.
From The Saker:
Sometime in the future a learned academic will be writing a weighty tome with the title The Decline and Fall of the Western Empire. Perhaps the Contents Page will include, among others, twelve chapters with titles something like this: World War One. World War Two. Korea. Vietnam. Palestine. Iran. Nicaragua. Afghanistan. Iraq. Syria. Ukraine. Taiwan.