On the afternoon of May 8, 1854, a ragged bunch of armed men approached the border between Mexico and the United States, just south of San Diego. Just ahead, a detachment of U.S. soldiers waited on the other side of the border to arrest them, while a band of eager spectators gathered atop a hill to watch the whole thing.

But surrender would not be an easy task: A band of Mexican fighters blocked the first group’s path, demanding the men give up their weapons before they would be allowed to cross the border. The U.S. soldiers stood back, refusing to intervene in any battle in Mexican territory—even one involving Americans.

However, the group seeking to cross back into the United States that day weren’t just any Americans. They were remnants of a conquering force that had invaded Mexico, in violation of U.S. neutrality law, and attempted to set up a republic in Baja California.

Their leader, a slight, blond, grey-eyed man from Tennessee named William Walker, would go on to command a far more successful invasion the following year in Nicaragua, even installing himself as that country’s president for a time.

Walker would become the most successful of the 19th-century filibusters, one of hundreds of intrepid Americans who set out with little more than weapons and ambition to conquer territory in Central and South America in the years leading up to the Civil War.

The practice of filibustering, or “freebooting,” took place without the consent of the U.S. government, in clear violation of the Neutrality Act passed in 1818, which banned attacks made from U.S. soil against nations with which the country was at peace. To make matters even more complicated, many filibusters were Southerners who sought to expand slavery’s reach into the territory they seized, exacerbating tensions at home that would eventually explode into war.

As a young lawyer and newspaperman in New Orleans in the 1840s, Walker embraced the spirit of Manifest Destiny, which motivated and justified a sweeping U.S. expansion into new territories.

“There was this idea that white Anglo-Saxons were the ones who were going to civilize these areas that were not under America's sway at that point,” says Scott Martelle, an editorial writer for the Los Angeles Times and author of William Walker’s Wars: How One Man’s Private American Army Tried to Conquer Mexico, Nicaragua and Honduras. “I think with Walker, it was a mix of hubris, ambition and a kind of nascent white supremacy.”

In 1850, Walker moved west, to San Francisco, which had grown exponentially in the two years since gold was discovered at Sutter’s Mill. He later moved to Marysville, near Sacramento, where he began practicing law—and dreaming up a filibuster scheme of his own.

Walker set his sights on Mexico. Under the Treaty of Guadalupe Hidalgo, which ended the Mexican-American War in 1848, Mexico had ceded 525,000 square miles of territory to the United States. As the Mexican government lacked the men or resources to patrol the sparsely populated area south of the new border, “it was kind of a no-man's land...pretty much a lawless zone,” Martelle says.

Several French adventurers living in California had made efforts to settle in the territory, with permission from the Mexican government—which preferred to deal with the French rather than Americans. Thanks to opposition from Mexican ranchers and Native Americans living in the region, these efforts didn’t take, and Walker, then living near Sacramento, California, saw an opportunity.

“Walker thought in romantic terms,” Martelle says. “He was going to go and he was going to protect the settlers and civilize this area that Native Americans kept running roughshod over. He saw himself as kind of this altruistic figure, but also he had dreams of empire. He saw a power vacuum, in essence, and tried to move into it.”

In early 1852, he and several other men hatched a plan to get Mexican permission to form a mining settlement in the Mexican state of Sonora. When efforts to get permission failed, Walker decided to invade instead.

Dodging U.S. military officials in San Francisco, whom the federal government had tasked with blocking any filibustering operations on the West Coast, Walker recruited a team of fellow invaders, many of whom had missed out on the spoils of the Gold Rush and were looking for the next opportunity to strike it rich.

After U.S. General Ethan Allen Hitchcock ordered his soldiers to take possession of the ship that Walker was preparing for the expedition, Walker quietly found another ship, hustling 45 members of his ragtag army aboard and sailing under cover of night out of the harbor and past the Golden Gate on October 16, 1853. They arrived at Cabo San Lucas, near the tip of the Baja Peninsula, within two weeks, and by early November had captured La Paz, capital of the Mexican state of Baja California.

Proclaiming the region the Republic of Lower California (later changed to the Republic of Sonora), Walker named himself as president. Rather than adopt the laws and constitution of California, he instituted Louisiana state codes—which included slavery. The choice to legalize slavery in his republic was likely more practical than ideological.

“He needed a legal structure, and he knew that one,” Martelle argues. Though Walker favored states’ rights and thought territories should make their own decisions regarding slavery, he never owned slaves, and wasn’t a strong advocate for the institution itself, he notes.

While Walker’s invasion of Mexico angered both the Mexican and the U.S. government, it was extremely popular with the American public, who saw filibusters as champions of Manifest Destiny. Bands of eager recruits headed down to join his expedition, and by January 1854 he had around 300 men under his command. But enthusiasm did not make up for the mission’s lack of adequate supplies, or for Walker’s weaknesses as a leader.

Despite his complete lack of military background, “Walker thought he was a general, and a mastermind,” Martelle says. “But he had no idea what he was doing. It was a small conquering party of Americans, and they didn't know the territory or the people. One of the first moves they made was alienating all the locals.”

After the Americans raided a local ranch, stealing horses, beef and other provisions, the furious rancher helped muster a resistance force to oust the invaders. Meanwhile, the captain of Walker’s original ship decided to take off with the vessel, stranding the would-be president and his men in hostile territory.

By the early spring of 1854, Walker’s struggling army was disintegrating from the inside. “Men started deserting by the fistfuls,” Martelle recounts. “People would show up and turn around and leave in a few days.”

Despite the tattered state of his army, Walker decided to march into Sonora and challenge the Mexican government for control of that territory as well. By May, however, he was forced to abandon this plan and march northward to the U.S. border with his remaining force of fewer than 35 men. After a final standoff with the Mexican resisters at the border, the Americans surrendered to the detachment of U.S. soldiers in San Diego, ending Walker’s first filibustering expedition.

When Walker underwent trial in San Francisco for violating the Neutrality Act, a jury acquitted him in just eight minutes, showing the strength of U.S. public opinion in his favor. Within a year, he would launch another filibustering campaign in Nicaragua, with far more success.

Taking advantage of a civil war in Nicaragua, Walker aligned himself with one faction against the other, and in October 1855 led a combined American-Nicaraguan force of nearly 300 in capturing the former Spanish colonial city of Granada. Walker declared himself president, and the U.S. government even recognized him as such in 1856.

During his time in Nicaragua, Walker made powerful enemies, including U.S. shipping and railroad tycoon Cornelius Vanderbilt, who sent soldiers to overturn his government after Walker revoked Vanderbilt’s rights to ship through Nicaragua. Vanderbilt’s men joined forces from other Central American nations, including Costa Rica, Guatemala, Honduras and El Salvador, which feared Walker might target them next.

Forced from power in Nicaragua after less than 10 months, Walker received a hero’s welcome upon his return to the United States, especially in the South. In 1860, he launched yet another filibustering operation, this time to Honduras. But the British government, which had important colonies in Central America, showed no tolerance for his interference in the region. The Royal Navy captured Walker upon his arrival in Trujillo, and turned him over to Honduran authorities, who had him executed by firing squad in September 1860. He was 36 years old.

With the outbreak of the Civil War, filibustering largely died out, and Walker’s exploits would soon fade into obscurity. Most people today think of Manifest Destiny as the doctrine that inspired settlers to push further west, but for Walker and his fellow filibusters, getting as far as the California coast was never enough. They chased their dreams of conquest into Mexico and Central America too, leaving a trail of destruction behind them.