Strategy and tactics play a key role in determining victory and defeat. In 1940, when the German military decisively defeated the combined forces of Britain and France even though the material capabilities of the British and French armies were arguably equal or superior to those of the Germans, this was in large part because of the strategy and tactics employed by the German military as compared to the French and British militaries. More recently the ability of U.S.-led forces to defeat the Iraqi insurgency beginning in 2006, after three years of futile efforts, was due not to improvements in the weaponry of the coalition forces but to a change in strategy and tactics.

A strategy is an overall plan for winning a war as a whole or a campaign within the war. Tactics are the methods used to win individual battles and engagements. The optimal strategy and tactics for any given war, campaign, or battle always depend on the specific circumstances – the objectives of the operation, the capabilities of the two sides, the geography, what the other side is doing, and so on. However, most militaries have a set of principles that they use to guide the selection of strategy and tactics. These principles are based on understandings about what methods were most effective in past wars as well as beliefs about what is most likely to be effective in contemporary conflicts, given changes in technology and other aspects of warfare. The term for these principles is doctrine. Sometimes militaries codify their doctrine in publications that they make available to their personnel, but in other cases doctrine is purely an oral tradition passed on from senior personnel to their juniors. In the U.S. and Chinese militaries doctrine is codified in official publications, although in each military there are undoubtedly also unofficial doctrinal principles that exist as purely oral traditions. Even when there is standardized official doctrine, moreover, different militaries treat it differently. In the U.S. military, doctrine is regarded as flexible, and the commander's personal military judgment is said to take precedence over official doctrine. Other militaries are more rigid about following doctrine than the U.S. military is.