What was the main cause of the Fall of the Roman Republic?

Caesar: Always to be remembered

The Plebeians’ reaction to their hero’s assassination:

The people were outraged at Caesar’s murder. They gave Caesar’s body a grand public cremation in the centre of the city and THEN RAN RIOT! They chased the murderers out of Rome, killed some of them and burned their houses

Caesar had left the vast majority of his money to his adopted son Octavian . But when Caesar’s will was read out in public, the citizens of Rome found they each had been left money. (A bit of emotional reactions here, then…)

After Caesar's death, a civil war took place that would change Rome forever

If the conspirators of Caesar's assassination thought that his death would restore back the original Senate-governed Republic, they were wrong. Very, very wrong.
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Mark Antony vs.

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Octavian

Battle of Actium

The Battle of Actium was a civil war. There was the side that supported Mark Antony [Roman general under Julius Caesar in the Gallic wars; repudiated his wife for the Egyptian queen Cleopatra; they were defeated by Octavian at Actium (83-30 B.C.)] and there was the side that supported Octavian (Later to be known as Augustus). This war was fought during September in the year 31 B.C. and it was fought next to a Roman Colony near Greece.

Octavian was the adopted son of Caesar. The problem started when Caesar had a baby with Cleopatra named Caesarion. This was pressure enough, but then Mark Antony decided that Caesarion was the rightful heir and he declared this publicly. This started a lot of fighting in the senate but in the end, Mark Antony lost his power, and the senate declared war on him and Cleopatra

Unfortunately for him, while Antony was waiting for Octavian to arrive at the gulf of Actium, malaria broke out aboard the ships and killed a lot of oarsman, that with the fact that the supply lines had been cut and left Antony with a severely diminished crew, and the type of ships he had demanded a big crew to successfully manoeuvre.

When Octavian arrived he had fresh crews and small light ships that were easier to manoeuvre compared to the huge ships that Mark Antony had and as the battle progressed, Antony’s general (Delius) decided that he was going to switch sides when he foresaw how this was all going to end, so on he went to Octavian, taking Mark Antony’s battle plans with him.

Things only got worse from there and eventually Antony had to leave the protection of the shoreline and go after Octavian because his ships would constantly stay out of range. Soon, however, Cleopatra foresaw a mighty defeat as well,   so she fled. Mark Antony then too got on a small ship and managed to escape, following Cleopatra - abandoning his troops.  

Things only got worse. Soon a fatal mix-up in communication left Antony with the belief that Cleopatra had been killed and so he committed suicide. When Cleopatra received the news, she too did the same.

The war was won and Octavian became Augustus Caesar - first Roman emperor...