Complex sentence: The battle was an belated effort by the Confederate Army to seize Union occupied Helena to in an attempt to draw Union Forces from Vicksburg, Mississippi to relieve the city there from the a long siege lead by General Ulysses S. Grant.
Split into simpler sentences: The battle was an belated effort by the Confederate Army to seize Union occupied Helena. It was an attempt to relieve the city of Vicksburg, Mississippi from the long siege on the city by Union forces lead by General Ulysses S. Grant.