The principle in war is that it should be launched with the authorization of, and under the banner of, the Muslim ruler; it is imperative that the decision to declare war be based on his own reasoning and his subjects must obey him. A ruler is authorized to declare war due to his knowledge of evident and hidden matters, the consequences of actions and the interest of his people. For this reason, a ruler is authorized to declare war and agree to domestic or international treaties as soon as he assumes office. In turn, he does not issue decisions based on [personal] whims.
The Muslim ruler declares war only after consulting specialists in every relevant field such as technical and military specialists and political consultants who are indispensable to military strategy. The luminary al-Bahuti said in Sharh Muntaha al-Iradat: “It is prohibited to [launch an] attack without the ruler's permission because he is responsible for making the decision of declaring war. [This is because] he has access to all the information pertaining to the enemy. [His permission is mandatory] except if [Muslims] are taken by surprise by non-Muslim enemies and fear their threat. [Only] then is it permissible to fight the attackers without the ruler's permission because of the general benefit therein.