Ancient Rome was an Italic civilization that began on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to become one of the largest empires in the ancient world with an estimated 50 to 90 million inhabitants (roughly 20% of the world’s population) and covering 6.5 million square kilometers (2.5 million sq mi) during its height between the first and second centuries AD. In its approximately 12 centuries of existence, Roman civilization shifted from a monarchy to a classical republic and then to an increasingly autocratic empire. Through conquest and assimilation, it came to dominate Southern and Western Europe, Asia Minor, North Africa, and parts of Northern and Eastern Europe. Rome was preponderant throughout the Mediterranean region and was one of the most powerful entities of the ancient world. It is often grouped into “Classical Antiquity” together with ancient Greece, and their similar cultures and societies are known as the Greco-Roman world. Ancient Roman society has contributed to modern government, law, politics, engineering, art, literature, architecture, technology, warfare, religion, language and society. A civilization highly developed for its time, Rome professionalized and expanded its military and created a system of government called res publica, the inspiration for modern republics such as the United States and France. It achieved impressive technological and architectural feats, such as the construction of an extensive system of aqueducts and roads, as well as large monuments, palaces, and public facilities. By the end of the Republic, Rome had conquered the lands around the Mediterranean and beyond: its domain extended from the Atlantic to Arabia and from the mouth of the Rhine to North Africa. The Roman Empire emerged under the leadership of Augustus Caesar. 721 years of Roman-Persian Wars started in 92 BC with their first war against Parthia. It would become the longest conflict in human history, and have major lasting effects and consequences for both empires. Under Trajan, the Empire reached its territorial peak. Republican mores and traditions started to decline during the imperial period, with civil wars becoming a common ritual for a new emperor’s rise. States, such as Palmyra, temporarily divided the Empire in a 3rd-century crisis. Soldier emperors reunified it, by dividing the empire between Western and Eastern halves. Plagued by internal instability and attacked by various migrating peoples, the western part of the empire broke up into independent kingdoms in the 5th century. This splintering is a landmark historians use to divide the ancient period of universal history from the pre-mediaeval “Dark Ages” of Europe.