[philman_36]

"...the peoples the Romans fought still fought wars as single combat..."
Adopting the Roman form of formation fighting, seizing hardware and equipment also helped foreign armies defeat them.
Consider Hannibal. He had learned their tactics during the previous years of war against Rome. Though he got beat in the end he did put on a good war.

[ichabod1]

That's not quite right. Hannibal was using tactical formations when the Roman's didn't know much beyond how to march over someone with large numbers of troops. This is during the 2nd Punic War. After getting slaughtered at Lake Kasimere, the Romans adopted a strategy of absorbing the enemy, much as the Russians would do later.

It was Scipio Africanus who was with his father at his fathers death at Lake Kasimere, who studied HANNIBAL's tactics and turned them back on him and ended up conquering Carthage.

To me one of the grand lessons of history is this: No one likes to have their own tactics used against them.