Imperial Military
The Imperial military is the professional fighting force of the Empire. Service in the Imperial Forces is largely compulsory and discipline is high. Loyalties to the Empire is paramount and sacrifice for the greater cause is a high ideal.
There are primarily Naval Space forces and Ground Military units similar to the Republic and an enlisted force along with an Officer corps composed of primarily upper-class citizens and nobility. Nepotism in the higher ranks is rampant and loyalty to those above you is prized. The Doctrine of the Empire is overwhelming force, destruction of the enemy and completion of the enemy takes priority over any consideration of collateral damage or loss of life.
Sith and the Military
Sith are a privileged class in the Empire and they are the greatest weapons of the military. They add flexibility and power to the Imperial forces, greatly amplifying the destructive capabilities of the Empire. Despite these strengths the Sith do add some serious complications to the Imperial military doctrine.
Despite their strength and power Sith can be just as deadly to their own forces as the enemy often more so. A Sith can usually kill a citizen with little impunity if they have enough power and influence. This extends to members of the military who often suffer when a Sith is displeased. A battle does not go as expected and a twenty year veteran is executed to sate the Sith’s anger. Sith play games of politics with the military and often turning their forces against each other and often countermanding orders with little thought of the military objective..
Sith technically outrank all military but in practice this tends to only work with low ranking soldiers and officers. Higher ranking officers have Sith allies and masters who do not like their assets toyed with. Coupled with the demand for loyalty and obedience and it means that Imperial forces are often subject to the whims of the Sith.