Battle of ActiumThe Battle of Actium was the dividing line between when the Roman Republic ended and the Roman Empire began. After a naval battle between Octavian (Augustus) and Antony, the former decisively defeated the latter to become the sole Roman ruler. It marked the start of centuries of imperial rule and was the root of what we think of when the Roman Empire is mentioned today, making it the most important event in the empire's history.
Roman Conquest of BritainThe Romans conquered Britain in 43 CE, for the purpose of acquiring slaves and riches, but mostly for show. This invasion was very important for not only the Roman Empire but Britain as well. These invasions caused the Romans to overextend their imperial reach, which contributed to thinning out their military and weakening their empire. With the troops that were used on this operation being available, the Empire could have sustained more barbarian attacks and possibly continued to stay intact.
Constantine takes power and Christianizes the empireDuring Constantine's rule, he began making changes to the structure of Roman religion. Becoming the state religion in 380, it was also a very vital part to how the Roman Empire eventually fell. By including popes and church leaders in political affairs, governance was complicated. This combined with the fact that the introduction of Christianity flipped the Roman value system on its head, had lasting effects on Roman civilisation and was one of many important contributors to the fall of Rome.
The empire is divided between East and WestAfter Constantine's death in 337, people began to realise that the Roman Empire was becoming too big for one man to rule. It split into the Eastern and Western Roman Empires. Without the aid from the rest of the Empire, this separation caused the west to eventually succumb to barbarian invasions. This split caused irreparable damage and marked the beginning of the Empire's true downfall.
Germanic InvasionsRome's financial security declined rapidly, which made the city very vulnerable. Raiders stripped the city of many valuables which in turn caused the people to rely less on the government and not pay taxes. The military would wane and with every invasion the empire grew weaker. This was a crucial series of events in the history of the Roman Empire as it paved the way for the final invasion that destroyed it once and for all.
The end of the Western EmpireHistorians state that the supposed end of the Western Roman Empire was when the final Emperor (Romulus Augustulus), was removed from power by Odoacer, who called himself the King of Italy. This ended a millennium-long Roman-ruled republic/empire and resulted in once-Imperial land being shared which evolved into the European countries we know today. Its culture and inventions resonate through and are used in our current society, and without this empire we would not live the same as we do today.