When did the Roman Army decline?

[Flavivs Aetivs]

The Decline of availible Manpower was due to several factors, and that decline didn't occur until the Invasion of 406 when Roman control of Gaul and Spain broke down, which was probably one of the biggest recruiting grounds as they still retained a militarized culture. People started looking to a career in the church rather than the Bureaucracy or the Military as well, and they preferred their local Bishop as a source of Leadership rather than their Dux Limitis.

Furthermore a decline in manpower does not mean a decline in Quality. Aetius' Army was predominately Limitanei drawn into the Field Army called pseudocomitatenses, and Aetius had the leadership quality to demand complete loyalty and a high level of discipline from his troops, which resulted in a successful army full of Veteran troops. Aetius never suffered a military reverse (except to Boniface in a Civil War) and a victorious army is one where recruits will flock to the standards