Consul Marcus Antonius started off as Julius Caesar's right hand. After his death in 44 by Brutus and Cassius, the battle for power would be won by the one who in the first place could present himself as the rightful successor to Caesar. Caesar had been able to count for support by the people of Rome and the soldiers were faithful to his name. Antonius, a political and military genius, knew this and tried to present himself as the natural successor to Caesar (to fob off the army) and to harmonize with the all-important Senate.
At Caesar's funeral, his will was read. It turned out that he had adopted his first cousin removed by testament: Octavianus, who now was called Gaius Iulius Caesar Octavianius by the laws of Roman adoption. Of course, Octavianus called himself Caesar, but Antonius refused to see the 18-year-old boy as a threat to his call for power.
Still, some military successes by Octavianus (the battle at Mutina being the most important) forced Antonius into cooperation with Caesar's stepson in 43. The so-called Second Triumvirate was born, magister equitum Lepidus being the third party. This was a collective dictatorship, legislated for five years (and after that another five) by the people to "reorganize the state". One of the first victims of the Triumvirate was Cicero, who was hated by Antonius, and vice versa.
In 42, Antonius helped Octavianus get his revenge by beating Caesar's murderers Brutus and Cassius in two battles near Philippi (Thracia). Both committed suicide.
Antonius and Octavianus then ended the relationship with Lepidus, who was accused of cooperating with the only left enemy Sextus Pompeius. Antonius forced an agreement through Octavianus' throat in which the empire was split in two halfs: Antonius got the east plus Gallia Transalpina (France), Octavianus the rest. Antonius did this because this was the part with enormous unrest: it was the Sextus Pompeius area and there was a huge problem with 100,000 soldier veterans who all suddenly had become rightfully land owners.
Antonius underestimated Octavianus, because the man managed to solve the problems. Marcus Antonius himself came into contact with Cleopatra in his own eastern side of the empire. He had initiated the contact because of the Egyptian wealth, that could be usefull in a possible expedition against the Parthians. This expedition failed completely with 22000 murdered Roman soldiers. The morally and military desillusioned Marcus Antonius then got caught in Cleopatra's web of dynastical ambitions and intrigues.
Marcus Antonius' power in Italy was officially over in 33. Octavianus called himself Dux (Leader) of Italy and the people plegded to him in masses. Antonius was without a profession, a rebel. Octavianus started a war against Cleopatra and Antonius, in which their troups ran out on them in the bay of Actium. Cleopatra and Antonius saw no other way out than to kill themselves.