Peter Heather, The Fall of the Roman Empire: Chapter 9, "End of Empire"

Heather starts by batting down suggestions of neglect by the Eastern Empire. He argues -- look at those Sasanians over there! and those Huns! -- that Constantinople had a full ledger already, and did well to spare what they could when they could for the West.

That done, Heather recounts the East's final attempt to shore up the West, starting with the installation of a new emperor on the Western throne. (Sidonius is still the main source for events in the West in this period.) The Western general Ricimer had fallen out with his original partner Majorian and then found no support forthcoming for his hand-picked replacement, Severus, so he had to settle with the the Eastern emperor Leo. As a result, the well-connected Eastern general Anthemius was promoted to the purple.

The first order of business was taking out the Vandals in North Africa. Majorian had already given this a go in 461, but the Vandal king Geiseric had destroyed his fleet in Spain. In 468, the combined Empire put together another army and fleet for a landing near Carthage, but the Vandals caught the fleet in an unfavorable wind and defeated it with the use of fire ships.

With the failure of the expedition, the Western Empire had run out of options. There weren't enough resources left to contain the barbarians in Western Europe. The Visigoths seized most of Spain and Southern Gaul, the Franks occupied northern Gaul, and the Burgundians and other tribes picked up smaller slices.

Heather examines the process of collapse though two examples -- one, Noricum, at the periphery, and the other, Gaul, in the core of the empire. In Noricum, the archaeological and literary evidence (the latter coming from the Life of Saint Severinus) shows that for several decades before the final collapse there had been a withering away of the army garrisons, and the abandonment of scattered estates for walled Roman refuge towns. Over time, the refuge towns consolidated and moved further away from the river frontier as security deteriorated.

In Gaul, literary evidence (from the letters of Sidonius, in particular) shows that the conquering Visigothic and Burgundian kings sought and often got support from the Roman landowning elites. The new barbarian rulers needed the Roman landowners to maintain the estates (and possibly provide taxes) and they needed skilled bureaucrats from the landowning class to administer their states. In return, the kings were willing to let cooperative Romans keep at least some of their land.

In Italy, Ricimer fell out with and deposed Anthemius, setting off a further round of musical chairs with the imperial throne. In the meantime, the mostly barbarian Roman army of Italy was getting restive over not getting paid; Odovacar, a general of Sciri birth, took matters into his own hands by deposing the last emperor and distributing land to the soldiers in lieu of pay. (I gather from the notes that Heather's main source on these events is Procopius.)