In July 1791, just as Le Republicain was getting ready to print its first issue, the King, who'd try to run away from France with his family, was caught in Varenne, recognised because of his likeness to the profile printed on a coin. He was brought back to Paris, and the Assembly, much relieved that they wouldn't have to do anything truly radical, decided to keep him on as the head of a constitutional monarchy.
This roused the republicans, and Brissot, often their spokesperson, drafted a petition to have the king removed from office. The word spread and on 17 July a huge crowd gathered on the Champ-de-Mars for the purpose of signing the petition. The Champ-de-Mars, now the site of the Eiffel Tower, was then a large empty space which had been chosen to celebrate the first 'Fete de la Federation' on 14 July 1790.
As is often the case in large gatherings, a few detractors made trouble, there was a fight, and possibly a death or two. This was enough for the Assembly to declare court martial and the army, headed by La Fayette, fell onto the crowd with their bayonettes.
It is unknown how many died or were wounded, but the event is refered to as the Champ-de-Mars Massacre.
Both Sophie and Manon were in the crowd, both with their daughters. The Marquis de la Fayette, famous for his role in the American Revolution had been a close friend of Grouchy before her marriage to Condorcet, and was generally in sympathy with Brissot and his friends. This, of course was the end of the friendship.