Principles of war
Economy of Effort – Economy of effort is the judicious exploitation of manpower, materiel and time in relation to the achievement of objectives.
Flexibility – the ability to change readily to meet new circumstances – comprises agility, responsiveness, resilience, acuity and adaptability.
Cooperation – Cooperation entails the incorporation of teamwork and a sharing of dangers, burdens, risks and opportunities in every aspect of warfare.
Sustainability – To sustain a force is to generate the means by which its fighting power and freedom of action are maintained.
These principles of war are commonly used by the armed forces of Commonwealth countries such as Australia.

Soviet Union and Russia: Soviet adoption of the principles of war is considered a part of military art, and is therefore a system of knowledge that is 
the theory and practice of preparing and conducting military operations on the land, at sea, and in the air. As such it includes the following principles
High combat readiness
surprise, decisiveness and active seeking to secure the initiative
full use of all means of combat
coordination and interaction of all types and branches
decisive concentration
simultaneous attack in depth
full use of morale-political factor
firm and continuous command and control
inexorability and decisiveness during the mission
security of combat operations
timely restoration of troop combat readiness
The Soviet principles of military science, from Soviet AirLand Battle Tactics ISBN 0-89141-160-7. Similar principles continue to be followed in CIS countries.

Preparedness – The ability to fulfill missions under any conditions for starting or the conduct of war.
Initiative – Utilizing surprise, decisiveness, and aggressiveness to continuously strive to achieve and retain the initiative. Initiative, in this sense describes efforts to fulfill the plan in spite of difficulties. This is in contrast to the western usage of the term which means attacking (or threatening to attack) to force enemy reaction, thus denying his ability to act.
Capability – Full use of the various means and capabilities of battle to achieve victory.
Cooperation – Coordinated application of and close cooperation between major units of the armed forces.
Concentration – Decisive concentration of the essential force at the needed moment and in the most important direction to achieve the main mission.
Depth – Destruction of the enemy throughout the entire depth of their deployment.
Morale – Use of political and psychological factors to demoralize opponents and break their will to resist.
Obedience – Strict and uninterrupted obedience. Orders are to be followed exactly and without question. Commanders are expected to directly supervise subordinates in a detailed manner in order to ensure compliance.
Steadfastness –  Subordinate commanders are to carry out the spirit and the letter of the plan.
Security – Security complements surprise. All aspects of security, from deception and secrecy, to severe discipline of subordinates who through action or inaction allow information to fall into the hands of ourselves are to be vigorously carried out.
Logistics – Restoration of reserves and restoration of combat capability is of paramount concern of the modern, fast paced battlefield.
Thus it can be seen that in Military art, the Soviet and Western systems are similar, but place their emphasis in wildly differing places. Western systems allow more control and decision-making at lower levels of command, and with this empowerment comes a consistent emphasis. Offensive, mass, and maneuver principles for the western commander all place a sense of personal responsibility and authority to ensure these principles are followed by appropriate action. In contrast the Soviet system stresses preparedness, initiative, and obedience. This places more responsibility at the better prepared and informed centers of command, and provide more overall control of the battle.

United States: (Refer to US Army Field Manual FM 3–0)
The United States Armed Forces use the following nine principles of war:

Objective – Direct every military operation toward a clearly defined, decisive and attainable objective. The ultimate military purpose of war is the destruction of the enemy's ability to fight and will to fight.
Offensive – Seize, retain, and exploit the initiative. Offensive action is the most effective and decisive way to attain a clearly defined common objective. Offensive operations are the means by which a military force seizes and holds the initiative while maintaining freedom of action and achieving decisive results. This is fundamentally true across all levels of war.
Mass – Mass the effects of overwhelming combat power at the decisive place and time. Synchronizing all the elements of combat power where they will have decisive effect on an enemy force in a short period of time is to achieve mass. Massing effects, rather than concentrating forces, can enable numerically inferior forces to achieve decisive results, while limiting exposure to enemy fire.
Economy of Force – Employ all combat power available in the most effective way possible; allocate minimum essential combat power to secondary efforts. Economy of force is the judicious employment and distribution of forces. No part of the force should ever be left without purpose. The allocation of available combat power to such tasks as limited attacks, defense, delays, deception, or even retrograde operations is measured in order to achieve mass elsewhere at the decisive point and time on the battlefield.
Maneuver – Place the enemy in a position of disadvantage through the flexible application of combat power. Maneuver is the movement of forces in relation to the enemy to gain positional advantage. Effective maneuver keeps the enemy off balance and protects the force. It is used to exploit successes, to preserve freedom of action, and to reduce vulnerability. It continually poses new problems for the enemy by rendering his actions ineffective, eventually leading to defeat.
Unity of Command – For every objective, seek unity of command and unity of effort. At all levels of war, employment of military forces in a manner that masses combat power toward a common objective requires unity of command and unity of effort. Unity of command means that all the forces are under one responsible commander. It requires a single commander with the requisite authority to direct all forces in pursuit of a unified purpose.
Security – Never permit the enemy to acquire unexpected advantage. Security enhances freedom of action by reducing vulnerability to hostile acts, influence, or surprise. Security results from the measures taken by a commander to protect his forces. Knowledge and understanding of enemy strategy, tactics, doctrine, and staff planning improve the detailed planning of adequate security measures.
Surprise – Strike the enemy at a time or place or in a manner for which he is unprepared. Surprise can decisively shift the balance of combat power. By seeking surprise, forces can achieve success well out of proportion to the effort expended. Surprise can be in tempo, size of force, direction or location of main effort, and timing. Deception can aid the probability of achieving surprise.
Simplicity – Prepare clear, uncomplicated plans and concise orders to ensure thorough understanding. Everything in war is very simple, but the simple thing is difficult. To the uninitiated, military operations are not difficult. Simplicity contributes to successful operations. Simple plans and clear, concise orders minimize misunderstanding and confusion. Other factors being equal, parsimony is to be preferred.
Officers in the U.S. Military sometimes use the acronyms "MOSS MOUSE", "MOOSE MUSS", "MOUSE MOSS", "MOM USE SOS", and "SUMO MOSES" to remember the first letters of these nine principles.

According to a United States Government document from 2010, the rule governing targeting in a non-international armed conflict is the international humanitarian law which is commonly known as the laws of war. The United States government stated in an undated Department of Justice White paper entitled "Lawfulness of a Lethal Operation Directed Against a U.S. Citizen who is a Senior Operational Leader of Al Qa’ida or An Associated Force" that the four fundamental law-of-war principles governing the use of force are necessity, distinction, proportionality and humanity i.e. the avoidance of unnecessary suffering.
There is a debate within the American military establishment to adopt flexibility as the tenth principle of war. Frost argues that the concept of flexibility should be integrated with America's warfighting doctrine.  Americans soundly retort that flexibility is a given that pervades all aspects of each principle.
Many, however, hold that the principle of simplicity implicitly includes flexibility.  One of the oldest dicta states that the simple plan is the flexible plan.
In 2007, Armed Forces Journal published a proposal by LCDR Chris van Avery, USN, 12 New Principles of War, to completely overhaul and expand the U.S. principles of war from nine to thirteen. The article was subsequently forwarded to the Joint Chiefs of Staff by Air Force Chief of Staff General Moseley and an effort to overhaul current U.S. doctrine was initiated using Van Avery's framework.
In 2011, three new "principles of joint operations" were added to the cited nine principles of war. These principles are:

Restraint – to limit collateral damage and prevent the unnecessary use of force. Restraint requires the careful and disciplined balancing of the need for security, the conduct of military operations, and the national strategic end state.
Perseverance – to ensure the commitment necessary to attain the national strategic end state. The underlying causes of the crisis may be elusive, making it difficult to achieve decisive resolution. The patient, resolute, and persistent pursuit of national goals and objectives often is essential to success.
Legitimacy – to maintain legal and moral authority in the conduct of operations. Legitimacy, which can be a decisive factor in operations, is based on the actual and perceived legality, morality, and rightness of the actions from the various perspectives of interested audiences.
Together, these 12 concepts form the Principles of Joint Operations.