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.