Continue gaining Legion infamy or continuing aforementioned questlines.
Reward: Legion infamy
Activating the quest can be done through several means. Completing the main story line quests of any other faction (the NCR, Mr. House or Yes Man) can activate it, as can gaining infamy with the Legion. In the former case, the quest will activate as a warning. Completing one more mission will cause it to fail. If triggered by Legion infamy, it will activate and fail on the spot. It will only activate after you have been cleared of your Legion infamy (if applicable) following the completion of Ring-a-Ding-Ding!, or after dealing with the Great Khans during For the Republic, Part 2 as siding with the NCR does not require confronting Benny.
Once failed, the Courier's reputation with Caesar's Legion will immediately turn negative, which in turn may cause the appearance of Legionary assassins by triggering Caesar's Foe. The Courier is also proclaimed "in damnatio memoriae" (condemned in memory).
The Legion has spies everywhere. If you continue to work against its interests, you will be marked an enemy.
For aiding and abetting the Legion's enemies, you have been declared "in damnatio memoraie" - an enemy of the Legion in perpetuity. Many will seek your head as a trophy.
This quest could be described as a counter quest to Don't Tread on the Bear!, even though it is possible to trigger both at once without actually failing either.
Its possible to activate Beware the Wrath of Caesar by not confronting Benny at the Tops but still advancing through For the Republic, Part 2.