In my opinion, the Schlieffen Plan during the First World War. This was created by General Alfred von Schlieffen (1833-1913) of the German Empire in 1905, and was executed in the war's opening moments.

Some background info: This was Germany's operational plan to attack France through Belgium, after Russia mobilized its army near Germany's border. He identified France as Germany's most powerful enemy, and also believed that the most decisive battles would be fought in the west. This was mainly because Russia was not as advanced as other powers in most areas, including industrialization. His plan was to conduct a swift and powerful surprise attack on the west, capturing France, and coercing Great Britain to not enter the war, all within six weeks. Schlieffen believed that Russia would take six weeks to mobilize, so if it was ready and began attack, Germany's army would effectively be able to hold them off until the bulk of the army was finished fighting in the west. Sounds straight forward, but there are some obvious issues.

First, as it turns out France soon became ready for war. Although Germany moved through neutral Belgium and France retreated during the Battle of the Frontiers, the German army came at most, only within 20 miles of Paris, which led to the First Battle of the Marne. France set up a line along the Marne River while taxi caps furiously sent reinforcements to the front lines, effectively ending Germany's run. As a result, both sides soon began to dig into their territory, starting World War I's most infamous and deadliest trait: trench warfare.

Second, the Schleiffen Plan assumed that not only Russia would take six weeks to mobilize, but also that Germany could defeat France in six weeks, then use its railroad system to transport troops to Russia. As stated before, the First Battle of the Marne halted Germany's advance, creating a stalemate, so obviously this failed. On of the reasons for this, was because of a withdrawal of troops from the Marne to the eastern front - because Russia's army had mobilized faster and stronger than anticipated.

And third, seeing France could hold its own, Great Britain declared war on Germany on August 4, 1914.

Overall, Schlieffen's plan was daring, but it could only be successful at 100%. His miscalculation left Germany in an arguably vulnerable state, as it was left open in the east to Russia, which came with its own set of issues. But more importantly, it led to the start of trench warfare, which by proxy led to millions of deaths and a deadlock which would last most of the war.