In September 1792, Couthon was elected to the National Convention. During a visit to Flanders, where he sought treatment for his health, he met and befriended Charles François Dumouriez, later writing praises of him to the Assembly, referring to him as "a man essential to us." His relationship with Dumouriez caused Couthon to consider joining the Girondist faction of the Assembly briefly, but after the Girondist electors of the Committee of the Constitution refused Couthon a seat on the Committee in October 1792, he ultimately committed himself to the Montagnards and the inner group formed around Maximilien Robespierre - a man with whom he shared many opinions. Couthon became an enthusiastic supporter of the Montagnards and often echoed their opinions. At the Trial of Louis XVI in December 1792, he argued loudly against the Girondist request for a referendum. He would go on to vote for the death sentence without appeal. On 30 May 1793, Couthon was elected to the Committee of Public Safety, where he would work closely with Robespierre and Saint-Just in the planning of policy strategy and policing personnel. Three days after rising to this position, Couthon was the first to demand the arrest of proscribed Girondists. Growing unrest had been occurring in Lyon in late February and early May. By 5 July 1793, the National Convention determined the city of Lyon to be "in a state of rebellion", and by September, the Committee of Public Safety decided to send representatives to Lyon to end the rebellion. Couthon would be the representative that Lyon would surrender to on 9 October 1793. He was suspicious of the unrest in Lyon upon his arrival, and would not allow the Jacobins of the local administration to meet with one another, fearing an uprising. On 12 October 1793, the Committee of Public Safety passed a decree that they believed would make an example of Lyon. The decree specified that the city itself was to be destroyed. Following the decree, Couthon established special courts that would supervise the demolition of the richest homes in Lyon, leaving the homes of the poor untouched. In addition to the demolition of the city, the decree dictated that the rebels and the traitors were to be executed. Couthon had difficulty accepting the destruction of Lyon and proceeded slowly with his orders. Eventually, he would find that he could not stomach the task at hand, and by the end of October, he requested the National Convention to send a replacement.