According to historical legend, Marie Antoinette's cry of, "Let them eat cake!" was the straw that broke the camel's back during the French Revolution. The story goes that Marie Antoinette, Queen of France, was informed that her subjects were starving because they had no bread. She was so pampered and out of touch with the reality of life for the poor that she suggested that they eat cake instead, which is what she would have done if she were out of bread. Marie Antoinette was convicted of treason and executed in 1793, months after her husband, King Louis XVI, had suffered the same fate.
In reality, the phrase predates the reign of Marie Antoinette. Jean-Jaques Rousseau, a philosopher who paved the way for democracy and socialism, wrote of a "princess" who said, "Qu'ils mangent de la brioche," when she heard that the peasants had no bread. While brioche is not quite as extravagant as cake, the phrase basically has the same meaning. The story told by Rousseau served to illustrate the vast gap between the rich and the poor of his time, but it was written when Marie Antoinette was only a child and not yet Queen of France.
No one knows the real origin of the phrase "let them eat cake," but it may have been a rally against the exploitation of the poor, rather than a flippant comment revealing the speaker's ignorance. In 18th century France, bakers were required by law to sell brioche and other fancy breads at the same price as regular bread if the latter was out of stock. Therefore, the original statement may have meant "do not let the poor starve if plain bread is not available."
One biographer has claimed that Louis XIV's wife, Marie-Therese, was the first to utter "Let them eat cake," but it remains unclear whether the story is strictly factual or simply a metaphor of the decadence of the French aristocracy.