The war with the Prussians had created a panic amongst the revolutionaries in Paris, as the Prussian troops had managed to invade Verdun – the last fortress before the fortress in Paris. Many feared a huge defeat if the Prussians were successful in their plight to over turn the revolution. Many patriots and troops were called in by Marat, a powerful figure in the Commune. The commune feared the Prussians and called in for all patriots to take up arms even supposedly paying many people to take part in the killings of those involved in the contempt of the revolution on hand.
The actual massacre itself started after rumours arose that counter-revolution prisoners in the full prisons of Paris were plotting to escape and kill those for the revolution, including innocent people, to help the Prussians invade. This made Marat make a lethal order to execute all those in the prisons, which included priests and nobles who were suspected to have been against the revolution. During the September Massacre, between 1100 and 1400 of the 2600 prisoners were killed, however, only a quarter of them were actually priests and nobles who were accused of following the counter revolution, and the rest were common criminals who were killed unjustly.
Another reason as to why so many were killed during the 5 days, were because the killers were the Sans Culottes. The Sans Culottes thought that not enough had been done to enforce the revolution, and to protect the Jacobins in the city. They used the rumours somewhat as an excuse to help ‘normal people of Paris’, and in fact to protect both the revolution and themselves. They killed many innocent people during the 5 days in many inhumane ways, displaying their loathing of those who opposed the revolution. This passion and hate that dwelled amongst the Sans Culottes initially started the brutal killings, and were partly responsible for the September massacres to break out in 1792.
Overall, the reason as to why the September Massacres broke out the in 1792 was mainly due to the suspicion and fear caused by the invasion of French soil by the Prussians, which then caused rumours that prisoners in Parisian prisons were plotting tot escape and help the Prussians take over. This fear and suspicion caused the brutality of the Sans Culottes, the killers of the September Massacres led by the ruthless Marat, to kill many innocent people, under the claim that they were part of the counter-revolution.