Why did the Schlieffen Plan fail in ww1?
Schlieffen plan correctly predicted that the French would launch an attack into Germany at the outbreak of the war. The plan, therefore, called for strategic retreat along the French border while at the same time swinging though Belgium and encircling the French north-east of Paris. Von Moltke, who took over as the Chief of Staff of the German Army after Von Schlieffen's death wasn't willing to let the French occupy any German territory, even temporarily. When the war broke out, and the French launched their offensive into Germany, not only the Germans didn't retreat luring the French into a trap, they actually counterattacked pushing the French out of the trap. At the same time the German First Army on the far right wing of the German advance though Belgium and northern France exposed its right flank to 300,000 strong Paris Garrison and the newly formed French Sixth Army. General Alexander Von Kluck, unaware of the British Expeditionary Force (BEF) and the French Fifth Army advancing from the south against his left flank, wheeled to face the new threat and exposed a 30 mile wide gap between his Army and the German Second Army to his east. Allied air reconnaissance discovered the gap and the Allied high command exploited it on 6 September 1914 by pouring the BEF and the Fifth Army into it. Initially, the Germans were hoping to defeat the Sixth Army, but these hopes were dashed when the Sixth Army was reinforced from the Paris Garrison on the night of 7/8 September, many solders famously riding to the battle in taxis. (Actually, most went by truck and artillery went by train, but the Marne Taxi is now part of the French lore, just like the Alamo in the US.) By 8 September the German Second Army was in jeopardy of being encircled unless all five German armies west of Verdun withdrew behind Ainse River when they dug in. That battle, now known as the Battle of the Marne, stopped the German advance, marked the end of the Schlieffen Plan and start of the war of attrition which Germany couldn't win. Moltke suffered a nervous upon hearing of the allied attack and only quick work of his subordinates at the General Staff prevented total disaster.
The wheeling maneuver moved too slowly and The Miracle Of The Marne held the pivot point as both armies stretched towards the channel trying (and failing) to turn the flank.
No plan survives contact with the enemy. Supporters of the Schlieffen plan would argue that it wasn't followed, and that the right cross was pulled too early. That is true, thus making it impossible to determine why it failed, because it wasn't followed.