Hadith: War is deception

عن أبي هريرة وجابر رضي الله عنهما أنَّ النبيَّ -صَلّى اللهُ عَلَيْهِ وسَلَّم- قالَ: «الْحَرْبُ خَدْعَةٌ».
[صحيح] - [متفق عليه]

Jābir (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "War is deception."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

This Hadīth indicates the permissibility of using ploys and deception in the course of war with the disbelievers; as this would cause harm to the disbelievers and avert losses for Muslims. This is not frowned upon in the Shariah; on the contrary, it is something recommended and desirable. Ibn al-Munayyr (may Allah have mercy upon him) said: "The good war, which attains its complete objective, is the one involving deception, not confrontation, for confrontation involves danger, while deception achieves victory without risk." Treachery and violation of agreements between Muslims and their enemies do not fall under the permissible deception, however. Allah, the Almighty, says: {If you [have reason to] fear from a people betrayal, throw [their treaty] back to them, [putting you] on equal terms. Indeed, Allah does not like traitors} [Sūrat al-Anfāl: 58]. In other words, if you have an agreement with some people and you want to revoke it, you should notify them before going to war against them, so that both of you should become on an equal footing.