The Sicilian Expedition (415-413BC): The Peloponnesian War between Athens and allies and Sparta and allies reached a turning point with the Athenian expeditionary force to Sicily. Athens had aligned itself with Sicilian city states against Syracuse and after a request for aid mobilized the expedition. The goals of the expedition were unclear from the start and posed significant risks sending a large force far from home with enemies in their rear. Despite initial success due to unprepared defenders the force quickly lost momentum due to incompetence in a divided leadership structure. The defences of Syracuse held while a Spartan force showed up in numbers to assist. Eventually the entire Athenian land and sea force was killed or captured into slavery. This loss was a significant portion of the overall Athenian forces and turned the war against them. Soon new allies joined the Spartans and 10 years later Athens surrendered.
https://en.wikipedia.org/wiki/Sicilian_Expedition
The Spanish Armada (1588): King Phillip II of Spain has had enough of Queen Elizabeth I after England's conversion to Protestantism and interference in the Netherlands. He proposes a massive armada to invade England with the main goal of removing the Queen and re-establishing Catholicism. The Armada is troubled from the start being a logistical nightmare requiring troops from Spain to meet up with troops from the Netherlands and carry all required food and supplies. To make it worse Philip puts an aristocrat with no naval experience in charge. Despite the larger size and quality of the ships in the Armada the smaller and quicker English and Dutch navies are able to successfully harass the Armada and prevent the troop pick up. After a English fire ship scatters the docked Armand bad weather forces the Spanish to head north around Britain with many sailors ending up killed or captured on the coasts of Scotland and Ireland. Overall this resulted in the growth of Protestantism in Europe and making England a major naval power at the expense of Spain.
https://en.wikipedia.org/wiki/Spanish_Armada
Battle of Antietam (1862): This was an epic failure for both the Union and Confederacy in the American Civil War. Robert E Lee had taken his Army of Northern Virginia into Maryland in an attempt to take the war to the Union, cut off Washington, and achieve international recognition for the Confederacy. He was followed closely by Union General George McClellan's Army of the Potomac. The first failure was that of a Confederate Soldier who had left Lee's battle plans wrapped around cigars to be found by Union soldiers. The plans showed Lee had divided his army leaving them vulnerable to attack. McClellan then failed in taking advantage of this waiting almost a full day before attacking. This resulted in the Battle of Antietam, a stalemate that was the single bloodiest day in US military history. The Confederates were forced to retreat from Maryland and didn't set foot in the North again until 1863. The Union did achieve a strategic victory but the over cautiousness of McClellan prevented a knock out blow and early end to the war.
https://en.wikipedia.org/wiki/Battle_of_Antietam