thumb|right|upright=1.35|[[Battle of the Little Bighorn known as Custer's Last Stand.]]

A military disaster is the defeat of one side in a battle or war which results in the complete failure of the losing side to achieve their objectives, often with a high and disproportionate loss of life. The causes are varied and include human error, inferior technology, logistical problems, underestimating the enemy, being outnumbered, and bad luck.

Entries on this list are those where multiple sources dealing with the subject of military disasters have deemed the event in question to be a military disaster (or an equivalent term).

Ancient era

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  • Battle of Marathon (490 BC). A large Persian force was destroyed and routed by a smaller Athenian force.
  • Battle of Salamis (480 BC). A huge Persian fleet was defeated by a united Greek force.
  • Syracuse Expedition (415–413 BC). Alexander the Great annihilated a much larger Persian army, thus ultimately conquering the Middle-east.
  • Battle of Cannae (216 BC). Hannibal destroyed the 16 Roman and Allied legions led by Lucius Aemilius Paullus and Gaius Terentius Varro. In all, perhaps more than 80 percent of the entire Roman army was dead or captured (including Paullus himself).
  • Battle of Zama (202 BC). A Roman army of 34,000 under Scipio Africanus annihilated the Carthaginian army of 50,000 under Hannibal, thus bringing an end to the Second Punic War.
  • Battle of Carrhae (53 BC). Crassus with 40,000 soldiers marched into Parthia, expecting to be victorious, chose to march a direct route through the desert instead of the mountains of the north. He and his army were entirely annihilated by 9,000 Parthian soldiers.thumb| [[Battle of the Teutoburg Forest]]
  • Battle of the Teutoburg Forest (9 AD). The Emperor Valens was killed while Gothic heavy cavalry ambushed and decimated his Roman heavy infantry.

Medieval era

  • Battle of Tours (732). The Muslim Moors marched into southern France meeting no foes, until encountering the Christian Frankish forces led by Charles Martel at Tours. Despite the Moorish advantage over the Franks militarily, they were defeated decisively by the Franks.
  • Battle of Hattin (1187). Overconfident Crusader forces from Jerusalem became trapped in a waterless desert area, and thus became easy prey for the Muslim forces of Salah-ud-din (Saladin)
  • Battle of Agincourt (1415). A large French army with a large contingent of knights was defeated by Henry V's much smaller army.

16th century

  • Battle of Flodden Field (1513). John Burgoyne's British Army was captured after the battle by the Continental Army under Horatio Gates.

19th century

  • Charge of the Light Brigade (1854). (1905) During the Russo-Japanese War, the bulk of the Russian Baltic Fleet, under Admiral Zinovy Rozhestvensky, sailed over halfway around the world () in order to relieve the blockade by Japan of the Russian Far East Fleet at Port Arthur, but was mostly sunk, scuttled, or surrendered in action against an Imperial Japanese Navy fleet under Admiral Tōgō Heihachirō, which took minimal losses.

First World War

  • Gallipoli campaign (April 1915 to January 1916). A combined British, Commonwealth and French attempt to capture Istanbul became a stalemate on the Gallipoli Peninsula and was abandoned with 300,000 casualties.

Second World War

  • Fall of Singapore (February 1942). It was the largest surrender of Commonwealth troops in history and destroyed the linchpin of the American-British-Dutch-Australian Command. Although the Japanese invasion force was half of the size of the defending force, Japanese air attacks on the city and lack of water proved decisive. Prime Minister Winston Churchill considered it to be the worst defeat in British military history, and it's been credited for undermining British prestige in Southeast Asia.
  • Battle of Midway (June 1942).thumb|Center of [[Stalingrad after liberation in 1943]]
  • Battle of Stalingrad