The third century AD showed some remarkable development for the empire as well as a potential decline and collapse. Emperor Caracalla's edict in 212 AD known as the Antoninian constitution delivered Roman citizenship to every free man in the empire, starting the decline of the importance of the city of Rome and indeed of Roman Italy for the Roman Empire. By the mid-third century AD, Pax Romana had come to an end, and several crises hit the empire simultaneously in a period known to history as the Crisis of the third century. This period saw the empire exposed to rampant inflation, natural disasters, secession and civil wars taking places over several decades. The secessions ended with emperor Aurelian’s conquest of the secessionist Palmyrene Empire and Gallic Empire in 273 and 274 AD, as Aurelian unified a splintered empire which, according to many historians, should have been doomed. However, Aurelian was assassinated in 276 AD, sparking additional civil wars. The crisis came to an end only through the ascension of emperor Diocletian to the throne. The West suffered more heavily from these decades of crisis due to this distinction between the established and more prosperous Hellenised East and the younger colonized and Romanized West that had persisted. This cultural division of the Roman Empire into an eastern Greek and western Latin part became increasingly important in later centuries, leading to a gradual estrangement of the two Roman worlds.

Emperor Diocletian created the administrative system known as the Dominate to guarantee security in all endangered regions of his empire. The dominate was in comparison with the principate, a huge imperial bureaucracy, which laid the foundations for the power structure of the later Eastern Roman Empire. With this came a redistricting and reduction of Roman provinces. Diocletian scrapped any pretense of republican rule as the emperor moved away from formally being merely "Princeps" to being "Dominus" - Lord and master. Diocletian also formally finished the process of restructuring the empire, from being a colonial-styled empire ruled by Rome and Roman Italians in the first century AD to a larger imperial entity where the emperor's court was not bound to the city of Rome or Italy, now only one Roman imperial province among many. The norm of emperors being Roman Italians was first broken by emperor Trajan who came from Hispania. By the time of the Severan dynasty, most emperors were originating from outside of Italy. When Diocletian introduced his reforms, this had the effect of the Roman Senate formally lost its already declining imperial influence and became a de facto regional body of decision-making.

An early instance of the partition of the empire into East and West occurred in 286, when Emperor Diocletian appointed Maximian as augustus of the West. In 293 AD, the empire went through a devolution under Diocletian known as the Tetrarchy. The empire was split into four, with the two most prominent parts of the empire each ruled by one emperor (Augustus). Each co-emperor then appointed a young colleague as caesar to be a subordinate emperor with control over his own territory. The Caesar shared power with and would eventually succeed the senior partner. Each tetrarch was in charge of a part of the empire, with the divisions based on geographic regions. This devolution outlines the coming split 100 years later when the empire was permanently divided into an eastern and western empire. The tetrarchy however only lasted 20 years, as emperors quickly began fighting each other for power. The whole empire was eventually reunited by Constantine the Great in 324 after he ended the last of these civil wars.
Given the referenced text, what Roman Emperor gave Roman Citizenship to all free men?
Emperor Caracalla gave Roman Citizenship to all free men in the Roman Empire.