Roman Legion's discipline, and unity. Most armies were made up of warriors. A warrior is a fighting man who fights as one.The Roman Legion was an army of soldiers. A soldier is a fighting man who fights as a group.The Legionairre did not fight alone, and he did not try to gain individual glory.
Also, the Romans used auxiliary troops, non-Romans who filled roles that the heavy infantry focused Roman military could not fill effectively, such as archers, light skirmishers and cavalry. In the Second Punic War (218 BC to 201 BC) the strength of the Italians union became critical. This was the form of the Roman armies of Caesar, Augustus and Emperor Titus. They conquered their old enemy, the Gauls in France and the Celts of Britain. In the East they took over the old Hellenistic Kingdoms, Syria, Egypt and Judea. The makeup of the Roman military changed substantially over its history, from its early history as an unsalaried citizen militia to a later professional force. The Roman military was intertwined with the Roman state much more closely than in a modern European nation. The Roman Army was one of the most successful in the history of the world and its soldiers were rightly feared for their training, discipline and stamina. As a result, the army was a major player in Roman politics and maintaining its loyalty was an essential task for any Emperor.  The core of Rome’s military strength lay in the professionalism of their heavy infantry. A force that was organized and reorganized as it evolved and adapted to survive the assaults of its mortal enemies, and conquer the Western world. This was the form of the Roman armies of Caesar, Augustus and Emperor Titus. They conquered their old enemy, the Gauls in France and the Celts of Britain. In the East they took over the old Hellenistic Kingdoms, Syria, Egypt and Judea. The Roman army was the backbone of the Roman Empire and one of the most successful armies in world history. It was well-trained, well-equipped, and well-organized. In order to guard such a large empire, the army took advantage of well built Roman roads to move about the empire quickly.  For a large part of Rome's history, the Roman state existed as an entity almost solely to support and finance the Roman military. The Roman military was intertwined with the Roman state much more closely than in a modern European nation. In the East the Roman Empire continued on for another thousand years as the Byzantine Empire. The Byzantine armies continued to develop the late roman military model. They made excellent use of cavalry, particularly mounted archers and were at times able to recover portions of the old Western Empire. This was the form of the Roman armies of Caesar, Augustus and Emperor Titus. They conquered their old enemy, the Gauls in France and the Celts of Britain. In the East they took over the old Hellenistic Kingdoms, Syria, Egypt and Judea. There are a number of reasons. Perhaps the most important one was the Roman Legion's discipline, and unity. Most armies were made up of warriors. A warrior is a fighting man who fights as one. The Roman Legion was an army of soldiers. A soldier is a fighting man who fights as a group. The Legionairre did not fight alone, and he did not try to gain individual glory.  The Roman military was the most successful and powerful in history, dominating the Western world for over a thousand years. The size, strength and organization of their infantry force wouldn’t be equaled again for another thousand years. This was the form of the Roman armies of Caesar, Augustus and Emperor Titus. They conquered their old enemy, the Gauls in France and the Celts of Britain. In the East they took over the old Hellenistic Kingdoms, Syria, Egypt and Judea.
http://ancientmilitary.com/roman-military.htm https://en.wikipedia.org/wiki/Military_of_ancient_Rome http://www.pbs.org/empires/romans/empire/soldiers.html http://ancientmilitary.com/roman-military.htm http://www.ducksters.com/history/ancient_rome_army_legions.php https://en.wikipedia.org/wiki/Military_of_ancient_Rome http://ancientmilitary.com/roman-military.htm https://uk.answers.yahoo.com/question/index?qid=20071121094003AAVOZV5 http://ancientmilitary.com/roman-military.htm
why was the roman army so victorious