To lead men, you have to have some kind of swag, right? Not these guys. Somehow, they all got put in charge of armies and led those armies to horrific defeat. These are the guys who nobody makes movies about or builds statues of. They're the most inept military commanders ever.
After a successful tenure at the end of World War I, lifelong soldier Arthur Percival was given the biggest command of his career when World War II began. Percival was appointed the General Officer Commanding of Malaya, a vital keystone in Asia for British imports. Unfortunately, he wasn't up to snuff, and when the Japanese attacked, they decimated his forces, with just 30,000 Japanese troops destroying over 138,000 Allied soldiers. Winston Churchill called it the worst disaster in British military history.
Antonio Lopez de Santa Anna
Santa Anna was one of the earliest of Mexico's military leaders, and all schoolchildren learn about his remarkable squandering of human life, first battling against Mexican independence from Spain and then, when the wind blew the other way, for it. Santa Anna was famous for always allying himself with the most likely winner. He's most famous for leading a garrison of 2,500 men against the vastly outnumbered Texans at the Battle of San Jacinto in 1836, and losing to Sam Houston and his men. "Remember the Alamo" was their rallying cry, and they captured Santa Anna the day after.
Some of the commanders on this list arguably did find some success, but it depends on your definition of the term. Before WWII, General Rodolfo Graziani was in charge of the southern front when Italy took Ethiopia from the native tribesmen. His tactics were ruthless, and he became known as the "Butcher of Ethiopia." However, when WWII started and it was time to step up and invade Egypt, Graziani balked. Mussolini ordered him to continue, though, and when he took his army into Egypt, the British completely annihilated him like it was nothing.
The Civil War saw a lot of dumb decisions made on each side, but General Ambrose Burnside was probably the biggest ninny in the Union. His list of failings is lengthy, but some notable ones include sending troops across a bridge over a river where Confederate snipers could take them out at leisure despite the river being shallow enough to walk over. This led to defeat at the Battle of Fredericksburg. Later on, he ordered his troops into a crater for some reason and watched helplessly as Dixie gunmen congregated around the outside and slaughtered them all. This became known as the Battle of the Crater, despite not being much of a battle at all.
Russian Cossack Semyon Budyonny is a great illustration of how generals need to adapt to survive. He had great success during World War I, but when the Russian high command put him in Ukraine to fight back the German invasion during WWII, he was easily defeated. Budyonny was notorious for believing that modern military hardware like jeeps and tanks was inferior to old-school cavalry. Despite the fact that his orders were to repel the Nazi forces and not retreat under any circumstances, Budyonny panicked in the face of the German mechanized horde and surrendered all 1.5 million men under his command.
William George Keith Elphinstone
Scottish-born soldier William Elphinstone started his military career off strongly, serving in the Napoleonic Wars and helping to give the tiny tinpot his final drubbing at Waterloo. Unfortunately, when the Queen saw fit to assign him to Afghanistan during the First Anglo-Afghan War, things turned very, very bad. Elphinstone was not a commanding figure—he was old, scrawny, and suffered from gout—and after he attempted to call a British retreat from Kabul in 1842, all 4,500 of his soldiers and 12,000 support staff were massacred. His colleague William Nott referred to Elphinstone after his death as "the most incompetent soldier who ever became General."
King Philip VI
It's always bad to have royalty in charge on the battlefield. Nobody can tell those guys what to do. An excellent example came at the Battle of Crecy during the Hundred Years War. After the English invasion of France, France's King Philip VI had his undies in a bundle. He personally accompanied his army to confront the Brits near the forest of Crecy. Upon sighting them, he sent his crossbowmen forward to attack. Unfortunately, the Brits were armed with longbows (much better range) and cut them all down. Undeterred, King Phil proceeded to send 15 cavalry charges that met the same fate. Total death count on the French side: 4,000.
It's telling that a lot of these lousy generals got their comeuppance in Africa. White dudes go there all the time, think they know everything, and then get their butts whipped hard. MacCarthy was Britain's governor over the Gold Coast region, and when he was tasked to put down the warring Ashanti tribe in 1823, his battle strategy got a ton of people killed. MacCarthy split his force into three columns and forced them to stand at muster when the Ashanti were more than 10 miles away. His ridiculous idea was playing "God Save The Queen" to scare them away. It didn’t work. His men were massacred, MacCarthy was captured and killed, and his heart was eaten by the Ashanti king.
One of the most important skills a general needs is the ability to read a battlefield, to know where the optimum strategic locations are and how to exploit them. James Abercrombie didn't have that ability. During the French and Indian War, Abercrombie had a force of 2,000 men with which to take Fort Ticonderoga. Nearby hills were perfect for setting up artillery, and splitting the force could flank the building, causing the defenders to panic. Abercrombie did none of that. He just marched his men up to the front door and watched as half of them were killed.
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Marcus Licinius Crassus
Another important rule of warfare: if you’re invading, don't listen to the locals. When Roman general Marcus Licinus Crassus took on the charge to Syria, he was already the richest man in the Empire. All his money didn’t buy him brains, though. He spent a year dicking around and then let his army be led into the desert by a traitorous Arab chieftain, who stranded Crassus' 43,000 men in the middle of the desert in 53 B.C. The Parthian army proceeded to ride in circles shooting arrows into the exhausted, thirsty Roman army, killing 20,000 men (including Crassus) and capturing 10,000 more.