alt Hacker News>If a premodern empire can field tens of thousands of iron-clad professional soldiers indefinitely, it must have a lot of civilian workers supporting that army.
Or it must continually conquer new territories to plunder and tax. And that was the Roman model. But finally the empire got too big to manage with the technology of the day. They weren't able to conquer new territories and that meant they could not afford the huge professiomal army. Which led to the collapse of the empire in the west. Or you could argue that it just morphed into the Roman catholic church.
(Not a historian, just been reading a bit about this recently)
With two exceptions (Britain and Dacia), the Roman empire mostly ran out of interesting neighbours to conquer by 1 AD.
Britain was considered Claudius' vanity project and probably was, on the net, an economic loss. Dacia was abandoned quite early precisely because it wasn't interesting enough to defend.
The legions remained pretty big long (centuries) after the expansion phase ceased.