The Greeks couldn’t fight off the Romans because no one could stop the Romans.
To borrow and slightly modify a famous quote from Napoleon “The Greeks’showed themselves worthy of victory, but the Romans showed themselves worthy of being invincible.”
The Romans were the most powerful fighting force the world had ever seen. There was simply no stopping their expansion.
Egypt, Macedonia, and even Carthage couldn’t stand against the might of the Roman war machine. Greece was no different. Rome was unstoppable.
During the Second Punic War the Romans quite literally went up against one of the greatest military strategists in history, Hannibal Barca.
Hannibal, a Carthaginian General, ravaged 3 Roman armies, killed off 20% of Rome’s male population, killed 2 Roman consuls in battle, and subjected Northern Italy to constant raiding for years.
Even with such astounding success, Rome still won the war.
Carthage was defeated, stripped of its power, and eventually absorbed into the Roman machine after the Third Punic War.
The Romans had many advantages over essentially all of their rivals. These included their excellent road networks, powerful navy, and well trained and equipped soldiers.
On top of this they also supported a massive population in Italy, and had incredible production output both in terms of food, and military supplies.
All of these things made the Romans a fearsome rival in any confrontation, but what made them even more powerful and ultimately unstoppable, was their adaptability, especially in the early years.
As an example, when the Punic Wars broke out, Carthage was the premier naval power in the Mediterranean. The Romans simply couldn’t compete with them, and were at a huge disadvantage when it came to their navy. By the end of the Punic Wars, however, Rome’s navy was unrivaled, and easily the most powerful in the Mediterranean. This was due to the Roman’s adaptability and their willingness to change their strategy.
The Greeks were able to defeat the Persians for a few reasons. They were able to utilize their better trained troops to inflict severe damage on the Persians at the Battle of Thermopylae. The Persians were far from home and had thin supply lines so it was difficult for them to maintain such a large army. After the defeat of the Persian navy at the Battle of Salamis the Persians were essentially forced to retreat because they couldn’t maintain supply lines without control of the sea.
The remaining Persians in Greece were defeated by the better trained and better supplied Greeks at the Battle of Plataea. I would attribute the Greek victory over the Persians to mostly their naval superiority, though other factors also came into play.
The Romans, however, as we discussed above had essentially unrivaled naval control of the Mediterranean after the Punic Wars. Unlike the Persians, the Roman fleet was actually well led, well equipped, and well built. The Greeks wouldn’t have been able to defeat it nearly as easily as the Persians.
Additionally the Roman legions were just as well equipped and trained as the Greeks. Though you could certainly make the argument that the Roman soldiers were superior to Greek soldiers.
So basically when we look at everything, the Romans had more people, greater production capabilities, a better navy, more efficient transportation networks, and better trained and equipped soldiers.
The Greeks didn’t have a whole lot of advantages against the Romans, with the home-field advantage probably being their most important. Therefore when the Romans invaded they met with incredible success, just like they did in nearly all of their other invasions in the early years.
Even if the Romans lost a war or two, their continued persistence, backed by unrivaled population growth and production meant that they could recover quickly from losses and eventually win a long run war of attrition.
During the Pyrrhic War starting in 280BC, the King of Epirus, Pyrrhus, met with a string of victories against the Romans, though at a significant cost to his own troops.
After one such victory he supposedly remarked something along the lines of “If we defeat the Romans one more time my army will be ruined”.
This is obviously in reference to the fact that even when the Romans were defeated in battle, they could still win the war. Every soldier Rome lost could be replaced by 2 more in a matter of months. For small nations like Epirus, soldiers were harder to replace and therefore every one was important.
As we saw in the Second Punic War, the Romans lost 20% of their male population, but were still able to bounce back and win the war.
The Greeks could have dealt significant damage to the Romans if they played their cards right, but I think no matter how well the war went for the Greeks, eventually the Romans would pull ahead due to their superior adaptability and greater population.