There are countless battles and military strategies throughout history that have failed, due in part to overconfidence or prior victories.
The first six weeks of the Franco-Prussian War saw French armies, convinced of their superiority following French victories in the Crimean War and the wars of Italian unification, enter a war with Prussia convinced that their weapons and tactics would easily defeat the Prussians. However, the Prussians had better weapons and tactics, and French arrogance and complacency led to catastrophic French defeats at Sedan and Metz.
Likewise, the Zulus' victory at Isandlwana led Zulus to believe they would easily wipe out the British defenders in the Battle of Rorke's Drift 1879 with ferocity and sheer numbers. Instead, the British won the battle and killed approximately 370 Zulu men.
Bad decisions made at and before the 1863 Battle of Gettysburg by the Confederates were, in part, due to the outnumbered-five-to-two victory at the Battle of Chancellorsville during the American Civil War.
Captain and Brevet Lt. Col. William J. Fetterman boasted during America's Indian Wars that, given "80 men," he would "ride through the Sioux nation". He had contempt for the Sioux's fighting ability and overconfidence in his own military prowess. In 1866, during Red Cloud's War, he and his army of exactly 80 men (including two volunteering civilians) were massacred to the last man by the Sioux. It was possibly the worst army defeat on the Great Plains until eclipsed by the disaster at Little Bighorn ten years later.
The initial defeats suffered by the United States in the Korean War, when units reassigned from occupation duty proved incapable of resisting the North Korean advance, may have been caused by victory disease.
In the 1993 Battle of Mogadishu, after a quick win in the Gulf War, American forces badly underestimated Somali militia fighters, nominally winning a Pyrrhic victory, but having to withdrew from Somalia as result.
Hitler's 1941 declaration of war against the United States has perplexed historians, since Hitler was not obliged to declare war, either by treaty or circumstance. (The U.S. had not declared war, nor had it made any overt attack on Germany.) Overconfidence has been proposed as an explanation, since the war's turning point against Germany, the Battle of Stalingrad, was still a year away, and forcing the U.S. into a two-front war may have proved too powerful a temptation for Hitler to resist. As it turned out, Nazi Germany found itself to be the priority enemy to defeat first in the war, although the US still had sufficient resources to fight Japan.
During World War II, 120,000 American troops and 5000 German troops (from German 275th and 353rd Infantry Divisions) prepared to wage the Battle of the Hurtgen Forest. However, more German reinforcements arrived and the U.S. Army eventually lost the battle.
The Battle of Pingxingguan during the Second Sino-Japanese War between Japan and China. After a series of easy victories against their opponents, the over-confident Japanese failed to take elementary precautions.
The crisis suffered by Israel during the Yom Kippur War. Israeli victories in the Six Day War had made them overconfident, and units on Israel's borders were unprepared for Arab attacks, although Israel narrowly avoided defeat, and ended up militarily victorious, with units deep within enemy territory.