The practice of states recognizes that warnings need not always be given. General warnings are more frequently given than specific warnings, lest the attacking force or the success of its mission be jeopardized. Warnings are relevant to the protection of the civilian population and need not be given when they are unlikely to be affected by the attack. 

When circumstances permit, advance warning should be given of attacks that might endanger noncombatants in the vicinity. Such warnings are not required, however, if mission accomplishment requires the element of surprise or the security of the attacking forces would be otherwise compromised. 

Where the military situation permits, commanders should make every reasonable effort to warn the civilian population located in close proximity to a military objective targeted for bombardment. Warnings may be general rather than specific lest the bombarding force or the success of its mission be placed in jeopardy. 

A warning need not be specific; it may be a blanket warning, delivered by leaflets and/or radio, advising the civilian population of an enemy nation to avoid remaining in proximity to military objectives. The “unless circumstances do not permit” recognizes the importance of the element of surprise. Where surprise is important to mission accomplishment and allowable risk to friendly forces, a warning is not required. 

There is no law of war requirement that a combatant must be “warned” before he or she is subject to the application of lawful, lethal force … [The opinion then cites Article 26 of the 1907 Hague Regulations and refers to Article 6 of the 1907 Hague Convention (IX).] Article 57, paragraph 2(c) of Protocol I Additional to the 1949 Geneva Conventions of 8 June 1977 contains a more relaxed but similar requirement, updating the two 1907 provisions while reconciling the slight difference between them. Although not a party to this treaty, the United States regards this provision as a re-codification of customary international law. The warning requirement cited above was for the purpose of enabling the civilian population to take appropriate steps to protect themselves from the collateral effects of attack of military objectives, or otherwise from the effects of war; it is not an obligation to warn combatants of their imminent attack. The exception to the warning requirement, relieving a commander from the obligation in cases of assault (stated more generally as “unless circumstances do not permit” in the 1977 Additional Protocol I) recognizes the legitimate use of the fundamental military element of surprise in the attack of enemy military forces in order to reduce risk to the attacking force and to increase its chance for successful accomplishment of its mission. 

In U.S. practice, bombardment warnings have often been general in their terms, e.g. advising civilians to avoid war-supporting industries, in order not to alert the air defense forces of an impending attack on a specific target. Such was the case in the Korean War.