I went into the breach for my king.

The poet André Chénier was guillotined because he was mistaken for his brother; then, informed of its mistake, the Commune guillotined the brother, too. Antoine Lavoisier, the scientist, requested a short stay of his execution in order to complete an experiment. “The revolution has no need of scientists” was the reply. One of the condemned was the eighty-year-old Marshal Duke de Mouchy, whose elderly wife did not understand what was happening. “Madame, we must go now,” her husband said gently. “God wishes it, let us therefore honor His will. I shall not leave your side. We shall depart together.” As they were taken from the prison, someone shouted, “Courage!” Mouchy replied, “My friend, when I was fifteen, I went into the breach for my king. At eighty, I go to the scaffold for my God. I am not unfortunate.”

The above is about the French Revolutionary in 18th century.

Q1: was Marshal Duke de Mouchy a famous figure in French history?

Q2: what's the meaning of breach here?

Thanks!

https://www.merriam-webster.com/dictionary/breach