Boris Johnson, current prime minister of the UK, recently claimed that the Roman Empire fell due to large amounts of uncontrolled immigration.
Now I am aware that this, probably false, claim was made to further his anti-immigration stance in politics. The claim obviously refers to the migrating Germanic tribes that entered the Roman Empire throughout its late period.
Said "barbaric invasions" combined with a "cultural decline" has long since been a favourite in pop history, a theory popularized by the British historian Edward Gibbon in his magnum opus "The History of the Decline and Fall of the Roman Empire".
However, I am aware that most contemporary historians have long since debunked this theory.
Another reason I have heard of is that the economic circumstances of the late Empire with vast and expanding inequality, worsening economic output and a corrupt bureaucracy and state lead to its fall. This is a theory I have heard espoused a few times by left-wing politicians comparing the current rising inequality in the contemporary USA (and West in general) to the late Western Roman Empire. I have to admit that due to my political biases that is one of the theories I find the most likely, but there are other theories still.
So another, more recent, theory holds that it's decline was a result of climate change. The Mediterranean climate cooled down, leading to harvest failures and widespread famine. I don't know much about the validity of this theory, but I am aware that supposedly the climate shifted a lot during the late Empire.
The last reason I have heard of is that it was a simple result of repeated military failures against external foes, compounded by internal strife (the late Empire had many civil wars and emperors who got that position through assassination and military victory as former generals) and an overstretched empire.
Of course, I have also heard all sorts of combinations of the above factors.
So the question then is: Why did the Western Roman Empire truly fell?
Source for the claim made at the beginning:
How accurate is the popular view that "uncontrolled immigration led to the fall of the Roman Empire"? written by u/Iguana_on_a_stick tackles 3 different works to try and tease out an answer to the question