Why did Octavian declare war on Marc Antony and Cleopatra?

Several reasons led up to the war between Octavian and Antony and Cleopatra, but the main reason was power. Both men were in a power struggle for supremacy in Rome. Antony, by openly entering into a political alliance with Cleopatra played right into Octavian's hands. Cleopatra herself, by seducing Antony, set herself up as a tool for Octavian to use in discrediting Antony. The power struggle continued and culminated in Octavian's victory at Actium.