Roman army was maybe the most efficient war machine ever! It dominated European battle fields nearly thousand years. I guess typical for Romans was good equipment, discipline, good organization and systematic practice. It is said that training of legionaries was like battles with no blood and battles like training where blood run. Meaning of the sentence is that Romans were so well trained that they did not get scared at all when they met enemy – they were always ready. It seems to me that when Roman army had good leaders like Ceasar they were unbeatable. But when arrogant generals led the army could happen like in Carrhae or Teuroburg. But good example is Gaius Julius Caesar. He did not loose any battles against his enemies. Always he was ready to set troops on right places and make sure that they were ready for everything. Once he withdraw against Pompei in their first class but in good order. Basically Roman troops could be divided in three different groups. The iron heart of it was legionaries. They were well equipped trained soldiers and first always Romans. They had of course cavalry which was more like tactical weapon. They they were auxiliaries which were either light armed troops (bowmen, slingers etc) or experienced builders who take care when bridge, walls, towers or something else was needed. Later on little bit more about Roman troops.
When Rome was just young and growing city state Alexander the Great planned a new crusade to west. There he would have met Carthagians and their future enemy – Rome. We know that this crusade never happened because still rather young Alexander died before it. Maybe poisoned. If he would have attacked to west from Greece maybe our history could be different. He could have beaten small Rome and wiped it away, but maybe Carthage could have been good opponent. Anyway it was the destony of Rome to grow world power and fine generals like Scipio Africanus, Caesar, Marc Antony, Agrippa, Germanicus, Trajan and Constantine changed the world.
Roman legionnaire was the iron heart of any Roman army. It had everything needed – short sword, special spear, big shield and protective helmet. When legionnaires fought together in one line – side by side – it was very difficult for their enemies to hit through that line and penetrate through them. This Roman line was compared as a circular saw! It was an iron line of shields where regularly short swords were stabbing forwards. Scattered enemy had no chance against it.