The French Revolution progressed as it did because of the intervention of other European nations and also by the seizure of the revolutionary initiative by radicals.
Austrian and Prussian diplomats issued the Declaration of Pilnitz shortly after the capture of Louis XVI and Marie Antoinette, indicating that they would intervene in France if necessary. They hoped this would calm the revolutionary spirit in France; however the French National Assembly had been taken over by young radicals who were determined to fight monarchy wherever they found it.
we will incite a war of people against kings….Ten million Frenchmen, kindled by the fire of liberty, armed with the sword, with reason, with eloquence, will be able to change the face of the world and make the tyrants tremble on their thrones.
War was declared on Austria, but French forces were promptly defeated. Fear on invasion incited greater patriotic fervor, and French nobility were arrested an summarily executed en masse.
The revolutionaries demanded a complete remake of French society, including a new calendar. When it appeared that things were truly out of hand and the revolution was out of hand, Maximilien Robespierre and the Committee on Public Safety instituted the "reign of terror" to preserve the revolution at all costs.
The intervention of foreign powers and the radical outburst of nationalism caused the revolution to exacerbate itself. Robespierre put it best when he said "the Revolution devours its own children."