Military science
Via the study of history, the military seeks to avoid past mistakes, and improve upon its current performance by instilling an ability in commanders to perceive historical parallels during battle, so as to capitalize on the lessons learned. The main areas military history includes are the history of wars, battles, and combats, history of the military art, and history of each specific military service.

Military strategy and doctrines: Military strategy is in many ways the centerpiece of military science. It studies the specifics of planning for, and engaging in combat, and attempts to reduce the many factors to a set of principles that govern all interactions of the field of battle. In Europe these principles were first defined by Clausewitz in his Principles of War. As such, it directs the planning and execution of battles, operations, and wars as a whole. Two major systems prevail on the planet today. Broadly speaking, these may be described as the "Western" system, and the "Russian" system. Each system reflects and supports strengths and weakness in the underlying society.
Modern Western military art is composed primarily of an amalgam of French, German, British, and American systems. The Russian system borrows from these systems as well, either through study, or personal observation in the form of invasion (Napoleon's War of 1812, and The Great Patriotic War), and form a unique product suited for the conditions practitioners of this system will encounter. The system that is produced by the analysis provided by military art is known as doctrine.
Western military doctrine relies heavily on technology, the use of a well-trained and empowered NCO cadre, and superior information processing and dissemination to provide a level of battlefield awareness that opponents cannot match. Its advantages are extreme flexibility, extreme lethality, and a focus on removing an opponent's C3I (command, communications, control, and intelligence) to paralyze and incapacitate rather than destroying their combat power directly (hopefully saving lives in the process). Its drawbacks are high expense, a reliance on difficult-to-replace personnel, an enormous logistic train, and a difficulty in operating without high technology assets if depleted or destroyed.
Soviet military doctrine (and its descendants, in CIS countries) relies heavily on masses of machinery and troops, a highly educated (albeit very small) officer corps, and pre-planned missions. Its advantages are that it does not require well educated troops, does not require a large logistic train, is under tight central control, and does not rely on a sophisticated C3I system after the initiation of a course of action. Its disadvantages are inflexibility, a reliance on the shock effect of mass (with a resulting high cost in lives and material), and overall inability to exploit unexpected success or respond to unexpected loss.
Chinese military doctrine is currently in a state of flux as the People's Liberation Army is evaluating military trends of relevance to China. Chinese military doctrine is influenced by a number of sources including an indigenous classical military tradition characterized by strategists such as Sun Tzu, Western and Soviet influences, as well as indigenous modern strategists such as Mao Zedong. One distinctive characteristic of Chinese military science is that it places emphasis on the relationship between the military and society as well as viewing military force as merely one part of an overarching grand strategy.
Each system trains its officer corps in its philosophy regarding military art. The differences in content and emphasis are illustrative. The United States Army principles of war are defined in the U.S. Army Field Manual FM 100–5. The Canadian Forces principles of war/military science are defined by Land Forces Doctrine and Training System (LFDTS) to focus on principles of command, principles of war, operational art and campaign planning, and scientific principles.
Russian Federation armed forces derive their principles of war predominantly from those developed during the existence of the Soviet Union. These, although based significantly on the Second World War experience in conventional war fighting, have been substantially modified since the introduction of the nuclear arms into strategic considerations. The Soviet–Afghan War and the First and Second Chechen Wars further modified the principles that Soviet theorists had divided into the operational art and tactics. The very scientific approach to military science thinking in the Soviet union had been perceived as overly rigid at the tactical level, and had affected the training in the Russian Federation's much reduced forces to instil greater professionalism and initiative in the forces.
The military principles of war of the People's Liberation Army were loosely based on those of the Soviet Union until the 1980s when a significant shift begun to be seen in a more regionally-aware, and geographically-specific strategic, operational and tactical thinking in all services. The PLA is currently influenced by three doctrinal schools which both conflict and complement each other: the People's war, the Regional war, and the Revolution in military affairs that led to substantial increase in the defense spending and rate of technological modernisation of the forces.
The differences in the specifics of military art notwithstanding, military science strives to provide an integrated picture of the chaos of battle, and illuminate basic insights that apply to all combatants, not just those who agree with your formulation of the principles.

Military geography: Military geography encompasses much more than simple protestations to take the high ground. Military geography studies the obvious, the geography of theatres of war, but also the additional characteristics of politics, economics, and other natural features of locations of likely conflict (the political "landscape", for example). As an example, the Soviet–Afghan War was predicated on the ability of the Soviet Union to not only successfully invade Afghanistan, but also to militarily and politically flank the Islamic Republic of Iran simultaneously.

Military systems: How effectively and efficiently militaries accomplish their operations, missions and tasks is closely related not only to the methods they use, but the equipment and weapons they use.

Military intelligence: Military intelligence supports the combat commanders' decision making process by providing intelligence analysis of available data from a wide range of sources. To provide that informed analysis the commanders information requirements are identified and input to a process of gathering, analysis, protection, and dissemination of information about the operational environment, hostile, friendly and neutral forces and the civilian population in an area of combat operations, and broader area of interest. Intelligence activities are conducted at all levels from tactical to strategic, in peacetime, the period of transition to war, and during the war.
Most militaries maintain a military intelligence capability to provide analytical and information collection personnel in both specialist units and from other arms and services. Personnel selected for intelligence duties, whether specialist intelligence officers and enlisted soldiers or non-specialist assigned to intelligence may be selected for their analytical abilities and intelligence before receiving formal training.
Military intelligence serves to identify the threat, and provide information on understanding best methods and weapons to use in deterring or defeating it.

Military logistics: The art and science of planning and carrying out the movement and maintenance of military forces. In its most comprehensive sense, it is those aspects or military operations that deal with the design, development, acquisition, storage, distribution, maintenance, evacuation, and disposition of material; the movement, evacuation, and hospitalization of personnel; the acquisition or construction, maintenance, operation, and disposition of facilities; and the acquisition or furnishing of services.

Military technology and equipment: Military technology is not just the study of various technologies and applicable physical sciences used to increase military power. It may also extend to the study of production methods of military equipment, and ways to improve performance and reduce material and/or technological requirements for its production. An example is the effort expended by Nazi Germany to produce artificial rubbers and fuels to reduce or eliminate their dependence on imported POL (petroleum, oil, and lubricants) and rubber supplies.
Military technology is unique only in its application, not in its use of basic scientific and technological achievements. Because of the uniqueness of use, military technological studies strive to incorporate evolutionary, as well as the rare revolutionary technologies, into their proper place of military application.

Military and society: This speciality examines the ways that military and society interact and shape each other. The dynamic intersection where military and society meet is influenced by trends in society and the security environment. This field of study can be linked to works by Clausewitz ("War is the continuation of politics by other means") and Sun Tzu ("If not in the interest of the state, do not act" ). The contemporary multi and interdisciplinary field traces its origin to World War II and works by sociologists and political scientists. This field of study includes "all aspects of relations between armed forces, as a political, social and economic institution, and the society, state or political ethnic movement of which they are a part". Topics often included within the purview of military and society include: veterans, women in the military, military families, enlistment and retention, reserve forces, military and religion, military privatization, civil-military relations, civil-military cooperation, military and popular culture, military and the media, military and disaster assistance, military and the environment and the blurring of military and police functions.

Recruitment and retention: In an all-volunteer military, the armed forces relies on market forces and careful recruiting to fill its ranks. It is thus, very important to understand factors that motivate enlistment and reenlistment. Service members must have the mental and physical ability to meet the challenges of military service and adapt to the military's values and culture. Studies show that enlistment motivation generally incorporates both self-interest (pay) and non-market values like adventure, patriotism, and comradeship.

Veterans: The study of veterans or members of the military who leave and return to the society is one of the most important subfields of the military and society field of study. Veterans and their issues represent a microcosm of the field.