Complex sentence: President Lincoln quickly rescinded it, reserving this "supposed power" to his own discretion if it were indispensible to saving the Union, and declaring Abolition to be outside the police functions of field commanders.
Split into simpler sentences: President Lincoln quickly rescinded it, reserving this "supposed power" to his own discretion if it were indispensible to saving the Union. Abolition was to be outside the police functions of field commanders.