Question: Gaius Cassius Longinus, was a Roman senator, a leading instigator of the plot to kill Julius Caesar, and the brother in-law of Marcus Junius Brutus, he commanded troops with Brutus during the Battle of Philippi against the combined forces of which Roman politician and general who played a critical role in the transformation of the Roman Republic from an oligarchy into the autocratic Roman Empire?

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Decimus Junius Brutus Callaicus: Decimus Junius Brutus Callaicus (or Gallaecus or Callaecus) (180 BC113 BC) was a consul of the Roman Republic for the year 138 BC together with Publius Cornelius Scipio Nasica Serapio.  He was an optimate politician and a military commander in Hispania and in Illyria.  He was the son of Marcus Junius Brutus (consul in 178 BC) and brother of Marcus Junius Brutus (praetor in 88 BC).  He had a son also named Decimus Junius Brutus (consul in 77 BC) and his grandson was Decimus Junius Brutus Albinus.

Liberators' civil war: The Liberators' civil war was started by the Second Triumvirate to avenge Julius Caesar's murder.  The war was fought by the forces of Mark Antony and Octavian (the Second Triumvirate members) against the forces of Caesar's assassins Marcus Junius Brutus and Gaius Cassius Longinus in 42 BC.

Gaius Cassius Longinus: Gaius Cassius Longinus (] ; October 3, before 85 BC – October 3, 42 BC) was a Roman senator, a leading instigator of the plot to kill Julius Caesar, and the brother in-law of Marcus Junius Brutus.  He commanded troops with Brutus during the Battle of Philippi against the combined forces of Mark Antony and Octavian, Caesar's former supporters, and committed suicide after being defeated by Mark Antony.

Lucius Junius Brutus: Lucius Junius Brutus ( ) was the founder of the Roman Republic and traditionally one of the first consuls in 509 BC.  He was claimed as an ancestor of the Roman gens Junia, including Decimus Junius Brutus and Marcus Junius Brutus, the most famous of Julius Caesar's assassins.

Assassination of Julius Caesar: The assassination of Julius Caesar was the result of a conspiracy by many Roman senators.  Led by Gaius Cassius Longinus, Decimus Junius Brutus, and Marcus Junius Brutus, they stabbed Julius Caesar to death in a location adjacent to the Theatre of Pompey on the Ides of March (March 15), 44 BC.  Caesar was the dictator of the Roman Republic at the time, having recently been declared "dictator perpetuo" by the Senate.  This declaration made several senators fear that Caesar wanted to overthrow the Senate in favor of tyranny.  The conspirators were unable to restore the Roman Republic.  The ramifications of the assassination led to the Liberators' civil war and, ultimately, to the Principate period of the Roman Empire.

Mark Antony: Marcus Antonius (Latin: ; January 14, 83 BC – August 1, 30 BC), commonly known in English as Mark or Marc Antony, was a Roman politician and general who played a critical role in the transformation of the Roman Republic from an oligarchy into the autocratic Roman Empire.

Battle of Philippi: The Battle of Philippi was the final battle in the Wars of the Second Triumvirate between the forces of Mark Antony and Octavian (of the Second Triumvirate) and the leaders of Julius Caesar's assassination, Marcus Junius Brutus and Gaius Cassius Longinus in 42 BC, at Philippi in Macedonia.  The Second Triumvirate declared this civil war ostensibly to avenge Julius Caesar's assassination in 44 BC, but the underlying cause was a long-brewing class conflict between the "ancien regime" represented in the Senate (Optimates), and the rising class (Populares).

Philippi (Rome): "Philippi" is the sixth episode of the second season of the television series "Rome".  The episode portrays the civil war between the Second Triumvirate and the combined forces of Marcus Junius Brutus and Gaius Cassius Longinus, murderers of Julius Caesar.  The Battle of Philippi is featured in this episode.

Epistulae ad Brutum: Epistulae ad Brutum (Letters to Brutus) is a collection of letters between Roman politician and orator Marcus Tullius Cicero and fellow politician, and conspirator against Julius Caesar, Marcus Junius Brutus.  The letters in this collection, when combined with Cicero's other letters, are considered some of the most reliable sources of information for the period leading up to the fall of the Roman Republic.  Cicero became acquainted with Brutus through his close friend Titus Pomponius Atticus, an admirer of Brutus.  Their personal relationship likely grew during their time together in opposition to Caesar during the civil war in 49 BCE, it being firmly established by the time Cicero returned to Rome in the autumn of 47.

Ptolemy XIV of Egypt: Ptolemy XIV (Greek: Πτολεμαῖος , "Ptolemaĩos", who lived 60 BC/59 BC–44 BC and reigned 47 BC–44 BC), was a son of Ptolemy XII of Egypt and one of the last members of the Ptolemaic dynasty of Egypt.  Following the death of his older brother Ptolemy XIII of Egypt on January 13, 47 BC, he was proclaimed Pharaoh and co-ruler by their older sister and remaining Pharaoh, Cleopatra VII of Egypt.  He and Cleopatra were married, but Cleopatra continued to act as lover of Roman dictator Julius Caesar.  Ptolemy is considered to have reigned in name only, with Cleopatra keeping actual authority to herself.  On March 15, 44 BC Caesar was murdered in Rome by a group of conspirators whose most notable members were Marcus Junius Brutus and Gaius Cassius Longinus.  Ptolemy soon followed him in death.  An inscription mentioning him as alive was dated at July 26, 44 BC.  It has been assumed but remains uncertain that Cleopatra poisoned her co-ruler, with aconite, to replace him with his nephew Ptolemy XV Caesarion, her son by Caesar who was proclaimed co-ruler on September 2, 44 BC and whom his mother intended to support as successor of his father.

Supporting paragraphs: Gaius Cassius Longinus Mark Antony
Answer: Mark Antony