After the murder of Caesar, Brutus and Cassius (the two main conspirators, also known as the Liberatores) had left Italy and taken control of all Eastern provinces (from Greece and Macedonia to Syria) and of the allied Eastern kingdoms.
The triumvirs decided to leave Lepidus in Italy, while the two main partners of the triumvirate (Antony and Octavian) moved to Northern Greece with their best troops (a total of 28 legions).