A political commissar, or politruk, is an officer appointed by a government to oversee a unit of the military. They are used by the government (usually after a revolution) to ensure that previously appointed officers and troops are loyal to the new regime.
They were first used after the French Revolution.
The term "political commissar" does not actually exist in the Russian language. The correct term is "military commissar" (военный комиссар).
While the institution of military commissars came to be associated with the Soviet period, in reality military commissars were introduced in the Russian Army by the Provisional Government in 1917. After the October Revolution, the institution of military commissars was adopted by the newly formed Red Army, where it lasted (with interruptions) until 1942.
It is important to understand the distinction between two different types of military commissars, who are related to each other in name only. The first type is not a political officer, but a territorial representative of the military authorities, responsible for registering males eligible for military service as well as organizing their conscription in a given territory. Such officer is commonly called "voenkom" (военком) in Russian, which is an abbreviation for "voennyi komissar" (военный комиссар), translated simply as "military commissar".
However, the institution of military commissars actually refers to the second type of military commissars -- the political officers. It is also important to understand that while all commissars in the Red Army were political officers, not all political officers were commissars. A commissar is a political officer whose position is equal to that of the commander of a given military unit and has the authority to countermand the orders of the commander. During the periods when political officers were completely subordinated to regular commanders, the institution of military commissars did not exist in the Red Army.
The position of military commissar was introduced in all units and formations from company (or equivalent) to division level, including naval vessels. Revolutionary Military Councils (RVS) were established at army and front (as well as fleet and flotilla) level, which consisted of at least three members -- the commander and two political workers assigned to him, but could be bigger as well. The political workers were referred to as "members of the RVS", rather than "commissars", even though they were a part of the institution of military commissars.
In 1919, the title of "politruk" (политрук -- political leader) was assigned to military commissars at company (or equivalent) level. While politruks were also a part of the institution of military commissars, they were not referred to as "commissars".
Starting in 1925, the course toward "edinonachalie" (единоначалие) (roughly, "single command") was set and the institution of military commissars was gradually abolished. Edinonachalie was introduced in two ways -- either the commander joined the Communist Party and also became the political officer, or the position of "pompolit" (помполит) (assistant commander for political work) was created under him. The pompolit was subordinated to the military commander and had no authority over him. In 1924, RVS's were renamed into Military Councils (political officers at that level became known as ChVS, short for Chlen Voennogo Soveta -- Member of the Military Council), and in 1934 they were completely abolished.
On 10 May 1937 the institution of military commissars was once again restored. Military Councils were created as well. This was connected with the purges that started in the armed forces. In August 1940 the position of military commissars was once again abolished. Military Councils remained and continued functioning throughout the war and after. Below army level the same system of "edinonachalie" was restored. In July 1941, following the defeats suffered by the Red Army in the beginning of the Great Patriotic War commissars appeared once again, as a move of desperation. As the war continued, it became apparent that they weren't helping, and the institution of military commissars was finally and for the last time abolished in October of 1942. Instead of the position of "pompolit", the position of "zampolit" (deputy commander for political work) was created from company (or equivalent) to regiment (or equivalent) level. Military Councils remained, but the final authority once again rested with the commander of the formation.
The position of political commissar has also existed and still exists in the People's Liberation Army of China. Usually, the political commissar is a uniformed military officer, although this position has been used to give civilian party officials some experience with the military. The political commissar was head of a party cell within the military; however, military membership in the party has been restricted to the lower ranks since the 1980s. Today the political commissar is largely responsible for administrative tasks such as civilian relations and counseling, and sometimes serves as second-in-command.