I think you are talking about the famous "Donations": in the gynmasium of Alexandria, Antonius ordered golden chairs to be erected on a high pedestal for himself and Cleopatra, and lower ones for the children they had together. He declared Cleopatra queen of Egypt, which had been nominally under the control of the Romans. This was quite a hazy case, but what came next was not: he also declared her queen of a part of Syria, and Cyprus. These were definitely places under the Roman thumb. Their little son Alexander Helios was to become ruler of Armenia and Parthia (which was still under the control of Parthians). Tripolitania was declared the dominion of the Alexander's little twin sister, Cleopatra Selene; and finally, their little baby son, Ptolemy Philadelphos, got the rest of Syria, Cilicia, and the Levant. There were all places under Roman control.

You ask why? The answer is not straightforward. Antony is putting on a show here for the benefit of his "eastern" "subjects." Alexander Helios was dressed like a Persian satrap, and little Ptolemy like a Macedonian soldier. Antony is sending a message about dynasty --the decayed Hellenistic type so unpalatable to the Romans (for the time being, anyway). Please know that this little production at Alexandria has zero bearing on the running of the empire. The complicated series of acta which governed these places were still in force, and Roman rule was still the norm. This was semantics, not binding decree. Neither Antonius nor Cleopatra had the sort of authority to issue such a decree, despite the fact that Antonius was, in all actuality, the dictator or tyrant of the eastern half of the Mediterranean. I suspect that this was all done for Cleopatra's reassurance, or perhaps even at her urging.

The alignment for the coming war was already set in place; these few years before the fighting began were spent in preparation. Octavian had about 45 legions under arms at this time, though many of them were agitating for discharge, and many others were unseasoned. He had some of them training in Illyricum to get that much-needed experience. Antonius, meanwhile, had the vast bulk of his forces distributed in the far far east, on the Armenian and Parthian frontiers. It would take time to turn that great war machine back towards the West, towards Italia. This was war on a scale never before seen in the Mediterranean. The amount of soldiers, ships, and money involved easily dwarfed all previous conflicts, including the Athenian invasion of Sicily in 415, the Punic Wars, or even the Civil War between Caesar and Pompey. Such conflicts take time to come about, take planning, take money. Please also remember that in the two years leading up to the conflict, both Octavian and Antonius were engaging in propaganda battles for public favor in Italy. Antonius would ultimately lose that battle, miserably, and in no small part because of his actions in Alexandria, and with whom he performed those actions. The "Donations" incident itself definitely played a role.