如火如荼 — “To be in full swing”

During the late Spring and Autumn period, King Fuchai of the State of Wu, after conquering several states, ambitiously marched northwest to defeat the State of Jin. However, King Goujian of Yue seized this opportunity, attacking Wu's capital, Gusu (Suzhou), and cutting off Fuchai's retreat at the Huai River.

Fuchai was shocked. His advisors suggested that retreating would mean double defeat, but defeating Jin would solidify his position as a hegemon. Deciding to employ a surprise tactic, Fuchai ordered his army to prepare.

Late one evening, 30,000 elite soldiers were chosen, forming three square formations. The central formation, led by Fuchai, was clad in white helmets, armor, clothes, and carried white flags and bows. The left formation was entirely red, and the right formation was all black. They departed at midnight, reaching within a mile of the Jin army by dawn.

As daylight broke, the Wu army's drums roared and cheers shook the earth. The Jin army awoke to a stunning sight: the white formation appeared 'as white as reed flowers (tu)', the red formation 'as red as raging fire', and the black formation seemed like a fathomless sea. This awe-inspiring display intimidated the Jin forces.

The idiom '如火如荼' (rú huǒ rú tú) originates from 'Guoyu: Wuyu' (Discourses of States: Discourses of Wu), and is used to describe the grand scale, powerful momentum, and enthusiastic atmosphere of a large-scale action, event, or movement.