Why and how did the French Revolution take a radical turn entailing terror at home and war with European powers?

When Louis XVI accepted the National Assembly’s constitution in September 1791, a young provincial lawyer and delegate named Maximilien Robespierre (1758–1794) concluded that “the Revolution is over.” Robespierre was right in the sense that the most constructive and lasting reforms were in place. Yet he was wrong in suggesting that turmoil had ended, for a much more radical stage lay ahead, one that would bring war with foreign powers, terror at home, and a transformation in France’s government.