Lets say that Augustus is assassinated as well when he returns to Italy, having the Senate feel he's a genuine threat to the Republic, being heir to Caesar. What happens next if Augustus is killed at Brindisi?
No. Marcus Antonius’ marriage to Cleopatra VII Philopátōr was a largely strategic alliance, a perceived necessity only after Antonius’ appointment as triumviri rei publicae constituent for the East, in which Cleopatra bankrolled Antonius’ disastrous Parthian campaign and Armenian war, as well as providing funding and military aid for the War of Actium. If Octavianus is assassinated by the Senate in collusion with liberatores, that will leave Marcus Antonius, in a alliance with the marshals of the Cæsarian faction -- Marcus Aemilius Lepidus, Munatius Plancus, Aulus Hirtius, and Pansa Cætronianus -- to establish a military junta and run roughshod over the res publica. Marcus Antonius was a incompetent, debauched, and lazy fool -- his massacre of Roman citizens in the forum Romanum while Cæsar was campaign against the Pompeians in Greece is probably an indication of how things would proceed. Antonius only emerged as the ally of the nobiles and the partisan of the Republic and libertas after Octavianus imposed his triumviral dictatorship in Italy and the western provinces. Without Octavianus, Antonius would still be seen by the aristocracy as the worst of the Cæsarian marshals. Most likely he would last a couple of years, and then be assassinated by a more moderate Cæ sarian with Republican sympathies -- Lepidus, or Calvinus possibly.Mark Anthony who was at least in the begining the senior partner takes Cesars place, marries Cleopatra and rules with an Iron Grip until maybe Agripa decides to challenge him.
Probably no Roman Empire for some time, the period of civil wars extended some 50-100 years. Potential for Rome to become a failed state.
Why do people also assume that in the exact decade of the 220’s AD, the Sassanids will automatically come to power in Persis and overthrow the Arsacid dynasty and the Parthian Empire? The rise of the Sassanids was actually due to set of highly specific causes -- namely, the prolonged Parthian decline since Trajan’s invasion in the 110s, the dissatisfaction of the Western Iranian peoples with the stagnant and unresponsive Parthian rule, the increasing autonomy granted so satraps and client-kings throughout the late second and early third centuries, the campaigns of Septimius Severus in the early 200’s, the civil war between Artabanus IV and Vologases VI, and, most importantly, the near destruction of the Parthian army at the prolonged battle of Nisibis in 217 ADAnd next, the Dark Age begins some centuries earlier in Western Europe, while the Persian empire (once it starts in 220 CE) will conquer the east of the empire, making Zoroastrianism the main religion?