In her book SPQR, the historian Mary Beard emphasizes that the Roman Empire was unable to conquer anything. The Roman Republic conquered the Mediterranean world, and then the Roman Empire failed at everything:
The only major, lasting conquest made by the Roman Empire was the conquest of Britain, under the Emperor Claudius. But for the most part, from the moment it was created, the Roman Empire was in a defensive crouch, trying to defend what the Roman Republic had built. The Republic had a culture that very much treated war as a glorious thing, and mobilized the public for total war, over and over again. The Roman Empire was very different, fighting became professionalized, and it became defensive.
https://www.amazon.com/Spqr-History-Ancient-Mary-Beard/dp/18...
The only major, lasting conquest made by the Roman Empire was the conquest of Britain, under the Emperor Claudius. But for the most part, from the moment it was created, the Roman Empire was in a defensive crouch, trying to defend what the Roman Republic had built. The Republic had a culture that very much treated war as a glorious thing, and mobilized the public for total war, over and over again. The Roman Empire was very different, fighting became professionalized, and it became defensive.