The union lost the battle of fredericksburg because

burnside ran from the fight rather than face lee

union forces were heavily outnumbered from the beginning

burnside was unable to adapt to changing circumstances

the confederate army was better trained as usual

The Union lost the Battle of Fredericksburg because Burnside was unable to adapt to changing circumstances.

Explanation:

The Battle of Fredericksburg, which took place in and around Fredericksburg, Virginia, from December 11 to 15, 1862, between the Northern Virginia Army of Confederate General Robert E. Lee and the Army of the Potomac commanded by the Major General Ambrose E. Burnside, is remembered as one of the most unequal battles of the American Civil War. The Union Army suffered terrible losses in useless frontal assaults on December 13 against the well entrenched and barricaded Confederate defenders in the hills behind the city, bringing an early end and serious defeat for the army of the Potomac and for the Unionist campaign against Richmond, the capital of the Confederation.

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