The Siege and Fall of Tenochtitlan 
A decisive occurrence in the Spanish Conquest of Mexico, the siege and fall of the capital of the Aztec Empire was aided by many factors – disease, politics, infighting by the Aztecs and ultimately, betrayal.
Only 350 Spanish conquistadors landed on the Mexican shores, but in less than two years, they were able to subdue and conquer the Aztec Empire, ultimately bringing about the fall of the civilization.
The major factor in their defeat was the ravages of smallpox brought to the Aztecs by the invading Spanish.
The Spanish Armada 
Spain was the 16th-century European superpower, and in her quest to overthrow the rule of Queen Elizabeth I of England she sent her armada, comprised of 130 ships, to invade England.
Unfortunately, though the Spanish Armada was massive, it relied heavily on an outdated way of sea combat and used gunnery only as a precursor to boarding. Furthermore, the fleet’s movement had a specific timeline which left many of the vessels waiting idly around; that or battling stormy sea weather.
In the end, Spain lost about 20,000 men. Most of them perished due to storms and ship worms rather than actual combat with the English.
The So-called Battle of Karansebes 
In this list, this is the only instance of an army defeating itself, with the factors of drunkenness and confusion playing major roles. This could arguably be the worst among the military disasters in this list.
The Austrian army, numbering to as many as 100,000 strong and able men, set camp near the town of Karansebes (now Caransebes which is located in Romania). The vanguard of the army crossed the river to scout for signs of the Ottoman Turks. The Hussars found none. Instead, they spotted a group of Gypsies who offered them alcohol, which they bought and drank.
Soon, a group of infantrymen crossed the river and, upon seeing the drinking party, demanded a share of the alcohol. The drunk hussars refused and set up a makeshift fortification around the barrels. Eventually, a heated argument arose and one soldier fired.
The rest of the army heard the gunfire, and chaos broke out. Thinking that an attack had begun, soldiers began to open fire at every shadow that moved, convinced that the Turks were closing in. Others fled the scene in terror.
It was only two days after this disaster that the real Ottoman Army arrived and found 10,000 dead and wounded Austrian soldiers. They easily took hold of Karansebes.
The Battle of Wabash 
The Battle of Wabash is, arguably, the worst military defeat suffered by America throughout its history.
In this event, American General Arthur St. Clair went ahead on a Midwest expedition despite internal problems within the troops he brought with him – there were mounting discipline difficulties and multiple desertions.
The Native Americans ambushed St. Clair and his group near the Wabash River and everything went downhill from there. St. Clair’s men just hid and fled despite their leader’s desperate and fruitless attempts to rally them.
In the end, out of the 1,000 soldiers who were with him, only 48 men survived.
The Battle of Adwa 
What are the odds that an African nation would defeat a European superpower? We would probably say, “none.” But 1896 was a different story.
It was the year that the Ethiopians defeated the Italians, allowing them to gain their independence as a state, and the defeat was all because of two military mistakes committed by the Italian Army’s leader.
For one, General Oreste Baratieri brought with him an army vastly outnumbered by the Ethiopians. Secondly, his forces brought with them cheap military equipment and insufficient maps.
Baratieri’s men marched through the night, got separated and couldn’t reorganize propely because their maps were useless.
So, the Italians were badly defeated in that battle and were forced to recognize Ethiopia’s independence. Not only that, Baratieri and his men left all of their military equipment behind when they retreated, and the Ethiopians captured this equipment for their own use.