This dissertation offers a new analysis of the activities of the Roman army in the Balkan peninsula between 200 BC, when the Romans declared war on Macedon and took a land army to Illyria, and 168 BC, when the Romans decisively defeated the Macedonians at the Battle of Pydna. This is derived from a close reading of ancient sources (primarily Livy, Polybius, and Plutarch) taken together with personal autopsy of the routes the Romans took in the modern countries of Greece, Albania, and FYROM. Chapter 1 covers the Roman campaign in the Myzeqeja plain during 200 BC. Chapter 2 focuses on the Roman campaign in the border areas between Illyria and Macedon during 199 BC. Chapter 3 covers the Battle of the River Aous, the first battle fought between the Romans and Macedonians, at the border of Epirus and Illyria in 198 BC. Chapter 4 covers Roman activities in Thessaly between 198-170 BC, including new reconstructions of the battles of Cynoscephalae in 197 BC and Thermopylae in 191 BC. Chapter 5 covers Rome's invasion of Macedon in 169-168 BC, culminating in the Battle of Pydna. The results of this dissertation point to a new way to interpret this period, organized around two different but related concepts: theater of war and Roman policy. During this period the Romans operated in three distinct theaters of war: the Myzeqeja plain and its surroundings, Thessaly, and Macedon. In turn, the transitions from one theater of war to the next coincided with the development of three distinct phases of Roman policy towards Greece: first to protect the Adriatic ports and gradually extend the buffer zone to the east, continuing the regional policy Rome had established and maintained since 229 BC; second to reduce the influence of each of the Hellenistic kingdoms while maintaining a balance of power in Greece; third to invade Macedon for the first time and permanently alter how it was administered. While the first two policies both acted to maintain some type of status quo in Greece, the third policy looked to actively change it.