Grasping the sheer scale and intensity of the Taiping Rebellion (1850-1864) - history’s largest and last pre-industrial war - is so hard.
The best analogy I can come up with:
Imagine if Mormon prophet Joseph Smith had stayed on the Erie Canal. His followers, armed mostly with spears and swords, rise up and capture the entire US Midwest, seizing the nation's agricultural breadbasket. They systematically slaughter anyone from New York City and starve out huge towns like Chicago. The war kills 5% to 7.5% of the population, double the actual US Civil War.
Then, right as the Federal government completely collapses, the British Empire sails steam-powered gunboats up the Hudson River. They arm a ruthless mercenary force with state-of-the-art rifles to crush the uprising: not to save the US, but to keep Wall Street and the Atlantic trade routes open.
Meanwhile, in a completely separate conflict(!), official British and French armies sail up the Potomac and loot and burn the Smithsonian, the Capitol and Mount Vernon.
The Taiping Rebellion killed 20 to 30 million people, more than WW1. It was World War China.