Bread and Circuses
Explanation
The Joke
The comic opens with the Latin phrase "Panem et Circenses" ("Bread and Circuses") and a man asking "What?" His companion explains it is an old Roman political concept describing how politicians give people food and entertainment so they remain passive while the powerful go about their villainy. The first man says this feels like what is happening now, and his companion insists he is "protesting" precisely because it is not happening -- there is constant outrage and no bread or circuses to pacify anyone.
A woman then asks, "We want bread and circuses?" and the protester clarifies that yes, they want them -- because at least then people would be getting something. He laments that people know history but are still doomed to repeat it, only to have the woman deliver the punchline: "Oh! Maybe that's why the Romans ate cake!" -- confusing the Roman concept with Marie Antoinette's famous (and apocryphal) quote "Let them eat cake," which was French, not Roman, and from a completely different era.
The Humor
The comic builds through several layers of irony. First, there is the funny realization that "bread and circuses" -- traditionally seen as a cynical tool of oppression -- might actually be preferable to the current situation where people get neither bread nor circuses but are still oppressed. The protester arrives at the paradoxical position of actually wanting the thing that is supposed to be bad. Then the final punchline deflates the entire intellectual discussion by having someone confuse Romans with Marie Antoinette, mixing up centuries of history and attributing "cake" to the wrong civilization entirely. This underscores the comic's theme about people failing to learn from history -- the woman cannot even keep her historical references straight while discussing the repetition of historical mistakes.
References
"Panem et Circenses" is a phrase coined by the Roman poet Juvenal in his Satires (circa 100 AD), describing how Roman politicians maintained public approval through free grain and spectacular entertainment. "Let them eat cake" (Qu'ils mangent de la brioche) is a phrase apocryphally attributed to Marie Antoinette of France during the French Revolution, roughly 1,700 years later.