Shalom. Where does it say that Jewish leaders did not learn the art of war, implying that they should have. Thank you.
ב"ה As Rabbi Blass pointed out in his answer, the Rambam in a letter to the scholars of Montpellier in 1195, (See Iggerot Harambam, Shilat edition , vol.II, pg. 480) makes the observation of the futile dependence of the kings in the past upon the star gazers and astrologers and not doing what they were supposed to learn the art of war. The Rambam infers the lack of study of war and conquering lands and dependence upon astrology from what the prophets referred to the leaders as "sechalim and evvilim", which can be translated fools and imbeciles. (See for example Yirmiyahu 4: 22 and,5:21.) The fact that leaders of nations need to train armies is something self- evident and doesn't need a source, because this is the way of the world. We can compare it to studying medicine. The Torah says that a person must not endanger himself and that he may be resort to a doctor for treatment but nowhere does the Torah say to train to be a doctor. The Torah says that one may not stand idly by when a brother is in danger, but the Torah does not command us to study forms of rescue. This is because these things are obvious. In order to give treatment, one must train to be doctor. In order to rescue people, one must be trained. A leader of a nation has to train an army to protect his country and needs medical and all other necessary services for the welfare of his people. It can also be inferred from the Torah principles of "pikuach nefesh", for the mitzvah of conquering Eretz Yisrael and other mitzvoth. Many also cite the source, of teaching "the sons of Yehuda archery" (See Shmuel B 1:18) as a source for training an army. However, in the context there the emphasis is particularly on the study of archery. This does not mean that the only way to wage war is archery, but as the Ralbag comments there, this was a tactical decision which needed to be made because the Jewish people were not trained well enough in that particular field of warfare. From here we can infer, that in today's time a Jewish army such as Tzahal, must be well trained in all forms of warfare so as not to give an edge to the enemy in any field and to counteract any possible threat. B"h it is well known today of Israel's specialty in the field of security and in the manufacturing of electronic and aeronautic equipment . All the best חג כשר ושמח