Ruling on learning worldly sciences and the manufacture of military hardware

With regard to science (apart from the sciences of sharee’ah), i.e., extracting metals and minerals, agricultural science and all beneficial sciences, it may be obligatory to learn of them that which the Muslims need, and it is fard kifaayah. The ruler should enjoin that which the Muslims need, and help the specialists in that field to achieve that, i.e., by helping them to do that which will benefit the Muslims and strengthen them against their enemies. A person’s deeds will be rewarded according to his intentions. It will be counted as an act of worship to Allaah if his intention is correct and sincere. If he does it without any intention it will be counted among permissible deed, I mean permissible kinds of manufacturing, extracting minerals, agriculture, etc… 

All of these are essential matters, but if they are done with the proper intention they may also be acts of worship. If there is no such intention then they may be permissible things. They may be fard kifaayah in some cases, if necessity calls for that. The ruler has to demand that of whoever is qualified to do it, because they are essential matters and there is a need for that. They differ according to intention and according to need.