The aim of this thesis has been to study the Roman art of war during the years 491-636. The primary aim has been to study the Roman tactics on land but the thesis also reasseses and studies the unorthodox fighting methods, strategy, siege tactics and naval warfare. The aim has been to assess the military effectiveness of the Roman army. The concept of military effectiveness is partially based upon the model created by Allan R. Millet, Murray Williamson, and Kenneth Watman. As a result of this thesis one can say that the Roman army was militarily effective at the strategic level when the empire was led by an able emperor. The Romans typically aimed at achieving numerical superiority at their own chosen theatre of operations. When this was not possible, they resorted to using guerilla warfare, diplomacy, allies, and defensive fortifications. Consequently, unlike usually assumed, the Roman field armies could be remarkably large even 30,000-50,000 men strong. This thesis has also shown that the Romans were also usually militarily effective at the tactical level. Its long military traditions and readiness to borrow what was useful from the enemies gave it greater flexibility that any of its enemies had. This did not ensure success on the battlefield but it did ensure them the ability to exploit the weaknesses that the enemies had. The Romans also possessed the most effective siege techniques. Its navy was the most effective fighting force in the Mediterranean area. The Roman cavalry was the most flexible fighting force of its era. It could fight at long range and in melee while mounted or dismounted. The Roman infantry was also equally adept at using missiles or melee weapons. The thesis has also demonstrated, unlike usually assumed, that the phalanx tactics were not inflexible or static. The units were able to operate independently. However, there were also serious weaknesses. The most important of these was the possibility for the cavalry to become separated from its infantry support if it pursued the enemy too far from its own infantry support. This weakness was partially responsible for the defeat at the Battle of Yarmuk in 636 and therefore important for the whole world history. As a result, the Muslims were ultimately able to spread even into Europe.