Feel like I can easily see how this could happen now. The military is hollowed out and becomes a corrupt patronage/welfare system to stave off civil conflict. Still some appetite for fighting but no ability to coordinate large scale, high intensity, or sustained conflict.
One bizarre question that arises from reading about the fall of the Roman Empire is: "Where the hell are all the soldiers?"
On paper, from documents such as the Notitia Dignitatum, the late Roman army was supposed to have had around half a million troops. Larger than ever before.
Yet time and time again we see barbarian invasions overwhelm Roman defenses, with an unclear military response. And when engagements do happen, the size of the Roman army reported is often smaller than during previous periods in which the total number of troops and manpower available to the empire was supposedly smaller.
So where was this vast Roman army when the Goths spent decades moving throughout the empire, or when the Rhine frontier fell in 406? Or when Rome was sacked and Britain was abandoned in 410? Or when North Africa was overwhelmed and lost? Did it just evaporate?