Final answer:
Robespierre and other radicals justified the use of terror to preserve the republic, prevent the king from seizing power, address fears of growing power of Jacobins and sans-culottes, and recruit people to fight against invaders.
Step-by-step explanation:
Robespierre and other radicals justified the use of terror in order to preserve the republic from its enemies, prevent the king from seizing power again, address fears of the growing power of the Jacobins and sans-culottes, and recruit people to fight against foreign invaders. The radicals believed that a period of violent repression known as the Reign of Terror was necessary to maintain republican ideas and restore peace.