On 10 February 1792, he gave a speech on how to save the State and Liberty and did not use the word war. He began by assuring his audience that everything he intended to propose was strictly constitutional. He then went on to advocate specific measures to strengthen, not so much the national defences as the forces that could be relied on to defend the revolution. Not only the National Guard but also the people had to be armed, if necessary with pikes. Robespierre promoted a people's army, continuously under arms and able to impose its will on Feuillants and Girondins in the Constitutional Cabinet of Louis XVI and in the Legislative Assembly. The Jacobins decided to study his speech before deciding whether it should be printed.
The Girondins planned strategies to out-manoeuvre Robespierre's influence among the Jacobins. He was accused by Brissot and Guadet of trying to become the idol of the people. On 26 March, Guadet accused Robespierre of superstition, relying on divine providence; being against the war he was also accused of acting as a secret agent for the Austrian Committee. When in Spring 1792, under pressure from the Assembly, the king accepted a few Girondin ministers into his cabinet, according to Louvet it was only due to a smear campaign by Robespierre and his followers that he was not also appointed. On 10 April, Robespierre resigned the post of public prosecutor, which he had officially held since 15 February. He explained his resignation to the Jacobin Club, on 27 April, as part of his speech responding to the accusations against him. He threatened to leave the Jacobins, claiming he preferred to continue his mission as an ordinary citizen.
When the Legislative Assembly declared war against Austria on 20 April 1792, Robespierre stated that the French people must rise and arm themselves completely, whether to fight abroad or to keep a lookout for despotism at home. On 23 April Robespierre demanded Marquis de Lafayette to step down. Robespierre responded by working to reduce the political influence of the officer class and the king. While arguing for the welfare of common soldiers, Robespierre urged new promotions to mitigate the domination of the officer class by the aristocratic and royalist École Militaire and the conservative National Guard. Along with other Jacobins, he urged in the fifth issue of his magazine the creation of an "armée révolutionnaire" in Paris, consisting of at least 20,000 men, to defend the city, "liberty" (the revolution), maintain order in the sections and educate the members in democratic principles; an idea he borrowed from Jean-Jacques Rousseau. According to Jean Jaures, he considered this even more important than the right to strike.
On 29 May 1792, the Assembly dissolved the Constitutional Guard, suspecting it of royalist and counter-revolutionary sympathies. In early June 1792, Robespierre proposed an end to the monarchy and the subordination of the Assembly to the General will. Following the king's veto of the Assembly's efforts to suppress nonjuring priests on 27 May, on proposal of Carnot and Servan in the Assembly to raise a (permanent) militia of volunteers on 8 June, and the reinstatement of Brissotin ministers dismissed on 18 June, the monarchy faced an abortive demonstration of 20 June. Sergent-Marceau and Panis, the administrators of police, were sent out by Pétion to urge the Sans-culottes to lay down their weapons, telling them it was illegal to present a petition in arms (to demand the king to apply the constitution, accept the decrees, and recall the ministers). Their march to the Tuileries was not banned. They invited the officials to join the procession and march along with them.
On 2 September 1792 French National Convention election began. At the same time, Paris was organizing its defence, but it was confronted with a lack of arms for the thousands of volunteers. Danton delivered a speech in the assembly and possibly referring to the (Swiss) inmates: "We ask that anyone who refuses to serve in person, or to surrender their weapons, is punished with death. Not long after the September Massacres began. Charlotte Corday held Marat responsible, Madame Roland Danton. Robespierre visited the Temple prison to check on the security of the royal family. The next day on proposal of Collot d'Herbois the Assembly decided excluding royalist deputies from re-election to the Convention. Robespierre made sure Brissot (and his fellow Brissotins Pétion and Condorcet) could not be elected in Paris. According to Charlotte Robespierre, her brother stopped talking to his former friend, mayor Pétion de Villeneuve, accused of conspicuous consumption by Desmoulins, and finally rallied to Brissot. On 5 September, Robespierre was elected deputy to the National Convention but Danton and Collot d'Herbois received more votes than Robespierre. Madame Roland wrote to a friend: "We are under the knife of Robespierre and Marat, those who would agitate the people."
In November 1792 Condorcet considered the French Revolution as a religion and Robespierre had all the characteristics of a leader of a sect, or a cult. As his opponents knew well, Robespierre had a strong base of support among the women of Paris. John Moore (Scottish physician) was sitting in the galleries, and noted that the audience was ‘almost entirely filled with women’. He is a priest who has his devotees but it is evident that all of his power lies in the distaff. Robespierre tried to appeal to women because in the early days of the Revolution when he had tried to appeal to men, he had failed. The Girondines called on the local authorities to oppose the concentration and centralization of power.
The convention's unanimous declaration of a French Republic on 21 September 1792 left the fate of the former king open to debate. A commission was therefore established to examine the evidence against him while the convention's Legislation Committee considered legal aspects of any future trial. Most Montagnards favoured judgment and execution, while the Girondins were more divided concerning how to proceed, with some arguing for royal inviolability, others for clemency, and others advocating lesser punishment or banishment. On 13 November Robespierre stated in the Convention that a Constitution which Louis had violated himself, and which declared his inviolability, could not now be used in his defence. Robespierre had been taken ill and had done little other than support Saint-Just, a former colonel in the National Guard, who gave his first major speech to address and argue against the king's inviolability. On 20 November, opinion turned sharply against Louis following the discovery of a secret cache of 726 documents consisting of Louis's personal communications with bankers and ministers. At his trial, he claimed not to recognize documents clearly signed by himself.
On 2 April the trial began on charges of conspiracy with the Duke of Orléans and Dumouriez. Corruption and a financial scandal involving the French East India Company provided a "convenient pretext" for Danton's downfall. The Dantonists were not serving the people. They had become false patriots, who had preferred personal and foreign interests to the welfare of the nation. "Danton had been a traitor from the beginning of the Revolution and the emergency law voted to stifle his resounding voice make this one of the blackest moments in the whole history of the Revolution." The defendants, of whom nine were députés of the convention, were removed from the room before the verdict was delivered. Fouquier-Tinville asked the tribunal to order the defendants who "confused the hearing" and insulted "National Justice" to the guillotine. Desmoulins struggled to accept his fate and accused Robespierre, the Committee of General Security and the Revolutionary Tribunal. He was dragged up the scaffold by force. On the last day of their trial Lucile Desmoulins was imprisoned. She was accused of organizing a revolt against the patriots and the tribunal to free her husband and Danton. She admitted to having warned the prisoners of a course of events as in September 1792, and that it was her duty to revolt against it. Remarkably Robespierre was not only their eldest friend but also witnessed at their marriage in December 1790, together with Pétion and Brissot.