Military doctrine
Defining doctrine: NATO's definition of doctrine, used unaltered by many member nations, is:

Fundamental principles by which the military forces guide their actions in support of objectives. It is authoritative but requires judgement in application.
In 1998 the Canadian Army stated:

Military doctrine is a formal expression of military knowledge and thought, that the army accepts as being relevant at a given time, which covers the nature of conflict, the preparation of the army for conflict, and the method of engaging in conflict to achieve success ... it is descriptive rather than prescriptive, requiring judgement in application. It does not establish dogma or provide a checklist of procedures, but is rather an authoritative guide, describing how the army thinks about fighting, not how to fight. As such it attempts to be definitive enough to guide military activity, yet versatile enough to accommodate a wide variety of situations.
A U.S. Air Force Air University staff study in 1948 defined military doctrine functionally as "those concepts, principles, policies, tactics, techniques, practices, and procedures which are essential to efficiency in organizing, training, equipping, and employing its tactical and service units".
A U.S. Army essay written in 2016 similarly defined military doctrine as "consist[ing] of tactics, techniques, and procedures (TTPs)".
In 2005 Gary Sheffield of the Defence Studies Department of King's College London/JSCSC quoted J F C Fuller's 1923 definition of doctrine as the "central idea of an army".
In 1965 the Soviet Dictionary of Basic Military Terms defined military doctrine as "a state's officially accepted system of scientifically founded views on the nature of modern wars and the use of the armed forces in them. ... Military doctrine has two aspects: social-political and military-technical." The social-political side "encompasses all questions concerning methodology, economic, and social bases, the political goals of war. It is the defining and the more stable side." The other side, the military-technical, must accord with the political goals. It includes the "creation of military structure, technical equipping of the armed forces, their training, definition of forms and means of conducting operations and war as a whole".

Development of doctrine: Before the development of separate doctrinal publications, many nations expressed their military philosophy through regulations.

United Kingdom: Field Service Regulations were issued by the War Office in 1909, 1917, 1923, 1930, and 1935. Similar publications under various names were subsequently published. Formal British Military Doctrine was first published in 1988 and in 1996 became British Defence Doctrine applicable throughout the armed forces.

France: The development of military doctrine in France came about in the aftermath of the nation's defeat during the Franco-Prussian war. The École supérieure de guerre, under the direction of its commandant, Ferdinand Foch, began developing a consistent doctrine for handling armies, corps, and divisions. Foch's 1906 work, Des principes de la guerre (translated by Hilaire Belloc as The Principles of War) expressed this doctrine.

Prussia and German Empire: Prussian doctrine was published as Regulations for the Instruction of the Troops in Field Service and the Exercises of the larger Units of the 17th June, 1870. The doctrine was revised in 1887 and published in English in 1893 as The Order of Field Service of the German Army, by Karl Kaltenborn und Stachau, and once again in 1908 as Felddienst Ordnung ("Field Service Regulations").

Soviet Union: Soviet doctrine was greatly influenced by M. V. Frunze.

United States: In the period between the Napoleonic Wars and the First World War, doctrine was defined by the War Department in "Field Service Regulations". In addition, many officers wrote military manuals that were printed by private publishers, such as Hardee's Tactics, used by both Confederate and Union forces. General George B. McClellan wrote a cavalry manual, Regulations and Instructions for the Field Service of the U.S. Cavalry, in 1862.
The General Staff became responsible for writing Field Service Regulations. They were published in 1908, were revised in 1913 and again in 1914 based on experiences of European powers in the first months of the war.
As late as 1941 U.S. Army doctrine was published in Field Service Regulations – Operations. This designation was dropped and replaced by U.S. Army Field Manuals (FM).

Relationship between doctrine and strategy: Military doctrine is a key component of grand strategy. 
NATO's definition of strategy is "presenting the manner in which military power should be developed and applied to achieve national objectives or those of a group of nations." The official definition of strategy by the United States Department of Defense is: "Strategy is a prudent idea or set of ideas for employing the instruments of national power in a synchronized and integrated fashion to achieve national or multinational objectives."
Military strategy provides the rationale for military operations. Field Marshal Viscount Alan Brooke, Chief of the Imperial General Staff and co-chairman of the Anglo-US Combined Chiefs of Staff Committee for most of the Second World War, described the art of military strategy as:
"to derive from the [policy] aim a series of military objectives to be achieved: to assess these objectives as to the military requirements they create, and the pre-conditions which the achievement of each is likely to necessitate: to measure available and potential resources against the requirements and to chart from this process a coherent pattern of priorities and a rational course of action."
Instead, doctrine seeks to provide a common conceptual framework for a military service:

what the service perceives itself to be ("Who are we?")
what its mission is ("What do we do?")
how the mission is to be carried out ("How do we do that?")
how the mission has been carried out in history ("How did we do that in the past?")
other questions.
In the same way, doctrine is neither operations nor tactics. It serves as a conceptual framework uniting all three levels of warfare.
Doctrine reflects the judgments of professional military officers, and to a lesser but important extent civilian leaders, about what is and is not militarily possible and necessary.
Factors to consider include:

military technology
national geography
the capabilities of adversaries
the capability of one's own organization

Military doctrine by country: 

China: Chinese military doctrine is influenced by a number of sources including an indigenous classical military tradition characterized by strategists such as Sun Tzu and modern strategists such as Mao Zedong, along with Western and Soviet influences. One distinctive characteristic of Chinese military science is that it places emphasis on the relationship between the military and society and views military force as merely one part of an overarching grand strategy.
According to French newspaper Le Monde, the Chinese nuclear doctrine is to maintain a nuclear force allowing it to deter and respond to a nuclear attack. However, new evolutions show that China could allow use of its nuclear arsenal in more situations.

France: Following the defeat of the French Army in the Franco-Prussian War, the French military, as part of its movements to increase professionalism, emphasized officer training at the École de Guerre. Ferdinand Foch, as an instructor, argued against the concept of a commander moving units without informing subordinates of his intentions. In doing so, a common doctrine served as a point of training.

We have then, a doctrine. All the brains have been limbered up and regard all questions from an identical point of view. The fundamental idea of the problem being known, each one will solve the problem in his own fashion, and these thousand fashions, we may very well be sure, will act to direct all their efforts to a common objective.Prior to WWI, France had an offensive military doctrine.
In the aftermath of WWI, France adopted a defensive military doctrine where the Maginot Line played a central role in its deterrence of Germany.

Germany: Prior to WWI, Germany had an offensive military doctrine exemplified by the Schlieffen Plan. Germany also devoted considerable resources to building a fleet of battleships, which provoked fears among European powers. During World War II, Germany deployed an operational strategy sometimes referred to as Blitzkrieg in its offensives against Poland and France.
German military doctrine incorporates the concept of Auftragstaktik (Mission-type tactics), which can be seen as a doctrine within which formal rules can be selectively suspended in order to overcome "friction". Carl von Clausewitz stated that "Everything in war is very simple but the simplest thing is difficult". Problems will occur with misplaced communications, troops going to the wrong location, delays caused by weather, etc., and it is the duty of the commander to do his best to overcome them. Auftragstaktik encourages commanders to exhibit initiative, flexibility and improvisation while in command.

India: The current combat doctrine of the Indian Army is based on the effective combined utilization of holding formations and strike formations. In the case of an attack, the holding formations would contain the enemy and strike formations would counter-attack to neutralize enemy forces. In the case of an Indian attack, the holding formations would pin enemy forces down whilst the strike formations attack at a point of Indian choosing.
India's nuclear doctrine follows the policy of credible minimum deterrence, No first strike, No use of nuclear weapons on Non-nuclear states and Massive nuclear retaliation in case deterrence fails.
India has recently adopted a new war doctrine known as "Cold Start" and its military has conducted exercises several times since then based on this doctrine. "Cold Start" involves joint operations between India's three services and integrated battle groups for offensive operations.