Day 69: Honorius (and Constantius III)
Honorius, the younger brother of Arcadius, became emperor of the west when he was ten years old. Yes, as you might imagine, he was quite the puppet emperor.
The most significant of his puppetmasters was a general named Stilicho, who was content to be the man behind the man largely because he was half-Vandal, and it would have been difficult for him to be emperor when the Vandals were one of the many Germanic tribes the west was so worried about. Given that disadvantage, though, Stilicho is also an example of how puppetmastery was not inherently bad--or, at least, if you're gonna have a puppet emperor, you should try to get the best guy around to pull on the strings.
The biggest problem during this initial period of Honorius's reign was Alaric, king of the Visigoths, one-time ally of the Roman army, and prime example of how maybe letting all those Goths just be inside the empire might not have been such a great long-term decision.
Alaric marched on Italy a couple times, but was pushed back by Stilicho--though, admittedly, not always by military means; once, Stilicho employed the time-honored Roman tactic of paying the enemy to go away.
Unfortunately, being a puppetmaster does not make you immune to political infighting, and Stilicho ended up dead in 408--the same year as Arcadius, actually.
The next guy behind Honorius, who by all rights should by now have been able to rule for himself, was a guy named Olympius.
Olympius did not do so good a job at protecting Italy.
Alaric saw an opportunity and took it.
Result: The city of Rome was sacked by a foreign enemy for the first time since 390 B.C. Check it: Eight. Hundred. Years.
The next truly significant puppetmaster was another general, Constantius, who was reasonably successful at fighting back the Germanic tribes... from Italy, at least. He also, very unusually for a puppetmaster, decided he didn't want to just be the power behind the throne anymore. He wanted to actually be an emperor, too. So he had Honorius name him co-emperor.
Theodosius II, the emperor in the east--or, more accurately, the people behind his throne--were not going to sit back and let that happen. They refused to acknowledge "Constantius III" as emperor. There would have been a war over it had Constantius not conveniently died.
Honorius died shortly after. By the time he died, the Roman empire had lost effective control over Britain and most of Spain and Gaul.