There are few bloodless events in American memory more dramatic than the 1848 discovery of gold at Johann Sutter’s sawmill in California’s Sacramento Valley. Schoolchildren are still encouraged to imagine the moment when a worker snatched an ingot from the water and, eyes bulging, shouted the four-letter word that fired men’s hearts: “Gold!” The other word most closely associated with the discovery — “rush” — described the perfect storm of madly competitive adventurers who flocked west to seek their fortunes, aided by a rainbow coalition of low-paid immigrant workers from both Europe and Asia.

The first piece of unearthed precious metal was no bigger than a pebble, maybe a bit softer than an ordinary stone, but otherwise unremarkable. It didn’t glisten until a laundress stewed it in a vat of homemade soap. When Sutter himself got hold of it, he patiently conducted his own experiments, taking directions from an encyclopedia.

That tiny gold piece turned out to be the tip of a mother lode, which really did lure thousands of frenzied “49ers” to the far reaches of the continent. They had to endure perilous sea voyages around the stormy tip of South America, or overland treks aboard wagon trains vulnerable to Indian attack. By either route, their horrific journeys could last up to half a year. Few got rich quickly, except the shipowners who transported the first migrants.

For the coming Civil War, all this turned out to be prologue. The important back-story of the Gold Rush, according to gifted historian Leonard Richards, is political and racial. Mr. Richards contends in this insightful new book, “The California Gold Rush and the Coming of the Civil War” (Knopf, 290 pages, $25), that for every fortune seeker who viewed California as a place to get rich discovering gold, another believed it a place to get rich exporting, utilizing, or trafficking in human slaves. California, Mr. Richards argues, loomed large and surprisingly long as a potential breeding ground for the expansion of slavery — “the best field for such labor now in America,” according to one frank Virginian. Many of the territory’s earliest settlers were Southern slave owners, including its first senator — William Gwin — who kept 200 blacks in chains in his native Mississippi.

It is hard to imagine today’s true-blue California as a hotbed of Southern sympathy, but Mr. Richards points out that in the 1840s and 1850s, it clung steadfastly to a pro-South orientation. Its dominant Democrats dubbed the “Chivalry Wing” for their undisguised Southern tilt. California banned slavery early, following an 1829 Mexican law, but even its most progressive early leaders campaigned to exclude free blacks as well. Admitted to the Union as a free state in 1850, its Congressional representatives nonetheless voted consistently with the pro-slavery South.

Alone among free soil states, California maintained a strong Democratic majority until the very outbreak of the Civil War, supporting future Confederate President Jefferson Davis’s dream for a Southern route for the proposed Transcontinental Railroad, and backing the divisive Kansas-Nebraska Act, which threatened to expand slavery into the newest Western territories.

At one point, California’s state legislature became so intent on replacing another of its initial U. S. Senators, anti-slavery “Pathfinder” John C. Frémont, that, unable to settle on a successor, it contentedly endured a vacancy for a full year. That left only one senator in Washington — the fascinating, slave-owning political schemer Gwin, a little-known, rather horrific American original whom Mr. Richards brings vividly to life.

Not surprisingly, Mr. Richards reminds us, when Frémont ran for the Presidency as a Republican in 1856, the state gave him an anemic 19% of the vote. Only in 1860, with Democrats hopelessly divided, did a Republican — in this case Abraham Lincoln — win this allegedly free state, and even then by just 657 votes (not 614, as Mr. Richards states), and less than 33% of the total. On the eve of that election year, a “Chiv” judge ended the career of the state’s lone pro-Northern Senator, a fellow Democrat, by killing him in a duel. California’s own civil war continued.

Mr. Richards never overtly claims that California was a microcosm of mid-19th-century America, but no reader will close his gripping book without reaching that conclusion. Bitterly divided over slavery, riven by immigration (Chinese, Irish, and Mormon), overrun by get-rich-quick adventurers, speculators, and patronage lords, pre-war California twice entertained proposals to split into two separate states, one free, one slave. When Lincoln became president, several California congressmen suggested that California declare itself an independent republic.

That never happened, of course. But Mr. Richards’s book should puncture the long-held myth of California as an isolated outpost where brave men quested only to make personal fortunes. In fact, it suffered from — and reflected — all the vexing problems that plagued antebellum America. The valuable gold in them thar hills made its divisive and often violent political struggles all the more crucial to the future of the country.

Mr. Holzer is the author of “Lincoln at Cooper Union,” which won a 2005 Lincoln Prize, among many other awards.