Oy. So many to choose from.
44 BC: Having killed Gaius Julius Caesar, the assassins return to the Senate and expect the Republic to be instantly returned to its glory days. Their failure to address any of the actual issues which allowed Caesar to become so powerful lead to Gaius Octavian following in the footsteps of Caesar, Sulla, and Marius to eventually make himself (in fact, if not in name) Emperor.
1863: General Jeb Stuart, incensed at having lost a skirmish with the Union cavalry, decides to reprise his glory days by riding his cavalry around the entire Union army. As a result, Lee has no cavalry during the coming battle of Gettysburg and has no idea where the Union troops are nor what strength they have, and his plan for an attack on the Union rear ends up actually hitting the center of the Union’s left line, and the Confederates lose the battle (and possibly their last chance of winning the war).
1864: Disgusted with General Joe Johnston’s constant retreating, CSA President Jefferson Davis fires him and replaces him with General John Bell Hood, stating, “This man will fight.” Hood immediately drops Johnston’s strategy of delay and replaces it with a strategy of ‘smash Sherman’s army’. This fails miserably, and while Hood is nursing his army back to health, Sherman proceeds to take Atlanta and begin his march to the sea. Hood eventually strikes out to hit Tennessee, where he will fight in the battle of Nashville and become the only general on either side of the war to lose his entire army. Johnston later points out that had he continued his delay strategy, Sherman never could have taken Atlanta by Election Day, and Lincoln likely would have lost his bid for re-election, thus making it possible that peace terms would have been reached and the CSA’s independence regonized.
1919: President Wilson leaves for the peace negotiations for ending World War I without bringing along a single Republican in his delegation; he expects the Republican controlled House and Senate will readily accept whatever treaty he comes up with simply by the moral force of his position.
1957: After Vietnam overwhelmingly elects Ho Chi Minh as President, US Secretary of State John Foster Dulles steps in and negotiates a partition of Vietnam so that the defeated candidate can have his own puppet-state to run.
1976: President Gerald R. Ford declares that there is no Soviet domination of Poland. Millions of listeners are stunned, and Ford loses re-election to Carter by a mere 10,500 votes (electorally, had 10,000 votes in Ohio and 500 in Hawaii switched, Ford would have won. He actually lost the popular vote by 1.5 million).