1. John C. Fremont

Abraham Lincoln may have freed the slaves in 1863, but the Civil War’s first Emancipation Proclamation actually came in August 1861 in the form of an unauthorized order from Union General John C. Fremont. A former frontier explorer nicknamed the “Pathfinder,” Fremont was a staunch abolitionist who had served as the anti-slavery Republican Party’s first presidential candidate. When he was given command of Union forces in Missouri, he immediately looked to strike a blow against Southern sympathizers. Without consulting his superiors, Fremont placed the entire state under martial law and issued an order freeing all slaves owned by citizens who backed the Confederacy.

The unprecedented decree made Fremont a hero of the abolitionist movement, but it also won him scores of political enemies, some of whom even spread rumors that he was planning to install himself as military dictator of an independent western state. President Lincoln, meanwhile, feared that prematurely freeing the slaves could cause Missouri to secede from the Union, and he requested that Fremont reverse his emancipation mandate. When the general refused, Lincoln removed him from his post and personally rescinded the order.

2. William Walker

A radical supporter of American “Manifest Destiny,” William Walker was the most daring of what were once known as “filibusters” or “freebooters”—bands of private citizens who carried out unauthorized military invasions of foreign lands. His illegal campaigns began in 1853, when he led a handful of would-be nation builders into the Baja Peninsula of Mexico. After capturing the city of La Paz, he proclaimed the region an independent country called the “Republic of Sonora” and established himself as president.

Walker’s upstart nation only lasted a few months before collapsing, but he soon took on an even more audacious expedition. In 1855, he led 57 men into Nicaragua, where they came to the aid of liberal revolutionaries in a bloody civil war. When the dust cleared, Walker had emerged victorious and even declared himself “presidente.” He would rule until May 1857, when an alliance of Central American countries besieged his army and forced him to surrender to the American Navy. Walker returned to the United States a populist hero, and later made three more attempts to reclaim his ill-gotten country. His infamous career finally came to an end in 1860, when he was captured and executed by a firing squad in Honduras.

3. Carausius

Rebel generals plagued the Roman Empire for much of its history, but few were as bold or as successful as Carausius. A skilled naval commander, he first rose to prominence around 286 A.D., when he was tasked with protecting the coastlines of Gaul from Frankish and Saxon pirates. While he kept the marauders at bay, Carausius was also known to pilfer their captured booty for himself, and his corruption eventually earned him a death sentence from the Roman Emperor Maximian. Rather than face the sword, the swashbuckling general marshaled his forces, withdrew to modern-day England and declared himself emperor of Britain.

After the Romans failed in an ill-planned attempt to retake the island, Carausius negotiated a flimsy peace deal that saw him recognized as the legitimate sovereign of Britain. Over the next few years, he built several coastal forts, issued his own coinage, and even conducted military operations in Gaul. He had plans to expand his empire deeper into the mainland, but his ambition was cut short in 293 A.D., when his finance minister Allectus engineered his assassination. Carausius’ unlikely empire would later collapse in 296 A.D., after Roman forces killed Allectus and reclaimed Britain.

4. Narciso Lopez

During the 1840s and 1850s, Venezuelan renegade Narciso Lopez led several highly illegal military expeditions to Cuba with the goal of inciting a revolt and wresting the island from Spanish control. A strong proponent of slavery, Lopez enlisted the help of powerful Southern politicians like Mississippi Governor John Quitman, who wished to see Cuba added to the Union as a new slave state. Lopez even nearly convinced Robert E. Lee to lead the invasion, but the future Confederate general ultimately refused.

Following a pair of abortive attempts to sail for Cuba, Lopez finally arrived on the island in May 1850 and occupied the coastal town of Cardenas. He had hoped his forces would be greeted as liberators, but his mercenary army came under immediate attack, and he only narrowly escaped to Key West with his life. Undeterred, Lopez launched a fourth invasion of Cuba in August 1851, this time with a band of 453 men-at-arms. He once again succeeded in landing on the island, but within two days his forces were routed by Spanish troops. Some 170 of the doomed army were captured, while another 50—including Lopez—were executed.

5. Roman von Ungern-Sternberg

In the early 1920s, Baltic cavalry leader Roman von Ungern-Sternberg launched a blood-soaked reign of terror in Russia and Mongolia. Nicknamed the “Mad Baron” for his psychotic behavior, Ungern was a fanatical monarchist who fought with the Tsarist “White Army” against the Bolsheviks during the Russian Revolution. Unable to stop the rising tide of communism, Ungern eventually went rogue and established himself as an independent warlord. In late-1920, he led his 6,000-strong cavalry into Mongolia, where he swept away occupying Chinese forces, captured the capital of Urga and reinstated the tradition ruler, the Bogd Khan.

Few would have questioned Ungern’s bravery, but the general turned heads with his bizarre and often tyrannical behavior. A devout Buddhist who surrounded himself with soothsayers and fortune tellers, he was also a rabid anti-Semite, and allowed his men to brutally rape and murder Urga’s Jewish and Chinese populations. Ungern believed his invasion would launch the second coming of Genghis Khan’s Mongol Empire, and he dished out medieval punishments to all who resisted his rule. These depredations would last until mid-1921, when Ungern fell into Soviet hands during a failed invasion of Russia. After a brief trial, the infamous “Mad Baron” was executed by firing squad.

6. Douglas MacArthur

General Douglas MacArthur was a hero of World War II, but he later clashed with President Harry Truman over his hyper-aggressive Korean War strategy. MacArthur took supreme command of a coalition of United Nations troops in 1950, after communist forces from North Korea invaded U.S.-allied South Korea. That September, in a stunning demonstration of tactical acumen, he executed a surprise amphibious landing at the city of Inchon and drove the communists beyond the 38th parallel.

MacArthur’s gambit put the North Koreans on their heels, but it also caused Chairman Mao’s China to enter the conflict in force. MacArthur—a staunch anti-communist—viewed this as an opportunity to escalate the war. He called for bombing raids on the Chinese, and even made inquiries about using atomic weapons. The general’s saber-rattling put him at odds with President Truman, who was hesitant to increase the scope of the conflict. The disagreement eventually intensified into a notorious war of words in which MacArthur repeatedly contradicted Truman’s presidential policy. Concerned that MacArthur’s public insubordination might compromise his authority over the military, Truman finally relieved him of duty on April 11, 1951.