How could Roman legions be defeated in battle? What were the weak spots?

They have the same weakness as any other army in history. This strategy has stopped the Romans, Mongols, Swedes and many other prominent armies in history. It would’ve stopped Alexander the great if the Persians weren’t so ignorant. It’s the nightmare of any invading commander. And it’s called the Scorched earth policy.

The scorched earth policy takes advantage of the fact that soldiers are men who need to eat and drink. The horses carrying their rations need to eat as well. The scorched earth policy takes away anything that is edible from the invading army.

Crops, villages, grass anything that would make the enemy continue to march forward would be scorched. After running out of their own supplies, the invading army’s morale drops and their men become weak and tired. The army is left with two choices. Pray to god and move on or retreat. And the better commanders chose the latter while confident or just plain stubborn commanders chose to move on. And their entire campaign ends up in a disaster. Prime examples of these commanders are Napoleon and Carolus Rex.

So if it can stop great commanders like them and their near unstoppable armies. It can stop the Roman legions as well.

Quora