One way of looking at it was that Roman Britain stopped enforcing immigration laws. They also couldn’t collect enough taxes to pay the Army, which either left or dissolved.
“One way of looking at it was that Roman Britain stopped enforcing immigration laws. They also couldn’t collect enough taxes to pay the Army, which either left or dissolved.”
Hadrian’s Wall was the ultimate in stopping immigration; I think it was more of the latter. While Rome conquered England, they never populated it; in the end, they left - leaving it to the people we see there today.
“One way of looking at it was that Roman Britain stopped enforcing immigration laws. They also couldn’t collect enough taxes to pay the Army, which either left or dissolved.”
The legions in Roman Britain were called one by one to return to Continental Europe to fight in the wars, foreign and civil, elsewhere in the Roman Empire.
At one point a civil war between claimants to the Roman imperial throne resulted in Rome’s loss of Gaul, and the lines of communication between Rome and Roman Britain were loss for a considerable time. During this period in which Roman Britain was relatively cutoff from Rome, the Germanic invaders increased their military threat against Roman Britain.
In response, Roman Britain rallied its society and defenses in part with the raising of a homegrown Roman legion recruited from among the cadre of legionaire retirees and the people of Roman Britain rather than the customary enlistees from the farthest reaches of the empire. No sooner had this new British Roman legion succeeded in stemming the tide of foreign invaders and raiders, when Rome reestablished contact long enough to order this legion to leave Britain to battle in the wars elsewhere in the empire. This left Roman Britain once again without an organized, trained, and experienced legion to serve in its own defense.