Short answers, pulled from the story.
Neoclassical architecture emerged in the 1750s as a radical reaction to the elaborate ornamentation of Renaissance and Baroque styles. This movement stripped away profuse decoration to emphasize planar qualities and simple geometry. The style aligned with the Enlightenment and replaced the imagination of the Rococo with empiricism and archaeological study.
Excavations at Pompeii and Herculaneum beginning in the late 1740s provided the raw material for a new aesthetic. These findings revealed genuine Roman interiors and inspired a vocabulary of flatter, lighter motifs and sculpted low frieze-like relief. The publication of Le Antichità di Ercolano Esposte by the 1760s allowed designers to recreate an authentically Roman interior vocabulary.
Richard Boyle, 3rd Earl of Burlington, designed Chiswick House in 1729 with William Kent to establish a severe lack of decoration. Colen Campbell published Vitruvius Britannicus in the first quarter of the 18th century to spread architectural prints inspired by Vitruvius to Palladio. The Adam brothers, Robert and James, further developed the style by publishing The Works in Architecture between 1773 and 1779.
The Empire style originated in France during the First French Empire to consciously idealize the leadership of Napoleon I. This phase was characterized by the work of court architects Percier and Fontaine and served as a grandiose wave of neoclassicism in both architecture and the decorative arts. The style marked a decline of the high-minded ideas that had inspired the movement's masterpieces.
Washington, D.C. was designed by Pierre Charles L'Enfant and revised by Andrew Ellicott in 1792 using the ancient Roman grid system. Karlsruhe and Saint Petersburg also adopted Neoclassical city planning patterns with central forums and main boulevards. The style influenced city planning in Buenos Aires and was used to transmit the presence of the State in various empires.