As Antony marched north to besiege Decimus Brutus in Cisalpine Gaul, Octavian, armed with the support of both Cicero and the Senate, readied his forces to follow. Having garnered the support of Cicero, though it was thought to be for the best interest of the Republic, Octavian actually secured his position as a political player of some importance. Despite attempts by the Senate to try and reconcile all the opposing factions, there was little chance now for resolution by peace. In the east, Caesar's assassins continued to spread their control, and in the west the various factions continued to build support for their own causes.
In April of 43 BC, Octavian marched north to face Antony, and was joined by the current consuls for the year, Pansa and Hirtius. The three men had mutual cause in defeating Antony, but otherwise the two consuls would have little use for Caesar's heir. The three commanders camped their armies separately near Bononia, not far from where Antony had Decimus Brutus besieged at Mutina. Antony broke off the siege with his main army, leaving his brother Lucius in command there. Antony drove down on Pansa first, defeating his Consular army, and inflicting what would become mortal wounds on Pansa himself. Next he sought to defeat Hirtius in turn, but the combined strength of two powerful enemies turned the tide against him. Antony retreated into Transalpine Gaul, where he hoped to build support from the Caesarian forces still in power there, but the armies of his enemies suffered a terrible loss. Not only had Pansa succumbed to his wounds, but Hirtius was killed in the battle as well, leaving the Republic without its Consuls. Decimus Brutus wanted Octavian to join forces with him and pursue Antony, but Octavian refused to join with one of the murderer's of his adoptive father.
While Antony fled and then joined forces with Lepidus, the pro-Caesarian governor of Spain, the Senate's faction attempted to shun Octavian and bestow prestige among Brutus and other allied parties. Among other slights, the Senate ignored land settlement requests for Octavian's veterans and refused to consider his petition to be named suffect Consul (along with Cicero) for the remainder of the year. While Cicero continued to support who he though was his young protégé, the assassins of Caesar saw the situation much more clearly. While Antony was a threat, to be sure, they feared Octavian, with his massive public popularity, as the continued presence of Caesar's legacy. Denying Octavian at every turn, the young man decided on another bold and daring move. Rather than wait for events to unfold, Octavian marched south, leaving Antony and Lepidus in a stalemate against Decimus Brutus and his allies.
Word had come to Octavian that the Senate and even Cicero, viewed him as a convenient tool to use against Antony. When the need was gone, the Senate would surely dismiss Octavian, but they underestimated both him, and his popularity with the legions. It's suspected that Antony may have even played a part, probably taunting Octavian with the fact that he was just an unwitting pawn in the Senate's power game.
As Octavian marched on Rome, the Senate was unable to put up resistance, with their main armies facing off against Antony and Lepidus in Gaul and the other 'Liberators' in the east. It's also interesting to note that while the assassins such as Brutus and Cassius participated in Caesar's murder as a pretense to preserving the Republican tradition, they seemingly has no qualms about illegally taking control of they eastern provinces to build their own strength. Regardless, the Senate initially accepted Octavian's demands, such as naming him Consul and granting land to his veterans, but when word arrived that legions had arrived from Africa to support the Senate, the offers were rescinded. These legions, however, refused to fight Caesar's heir, and switched their loyalty without a single engagement. As the calendar reached mid summer, Octavian, still only 19 years old, was confirmed as Consul along with his cousin Q. Pedius and set about an extraordinary bit of political legislation. He seized the state treasury, paying off his troops, and finally received the distribution of Caesar's will. More importantly, however, he changed the focus of Senatorial politics by authorizing a special law revoking the amnesty of the 'Liberators'.
In one single day, he tried and convicted Caesar's murderers in abstentia, and set stage for the civil war to finally end the collapsing Republic. Octavian rose to power as the first Roman in history to do so almost completely with the support of the army. Unlike previous political powerhouses, like Marius, Sulla and Caesar, Octavian had little to any real support among the political forces that were. As a young man without an established client base, the maneuverings of men such as Cicero helped set him up on the public stage, but it was the legions, and the use of his adoptive name, Caesar, that secured his position among Rome's elite. By the autumn of 43 BC, Octavian prepared for his next move, to alter the state of the political landscape and reconcile with Antony.