Gemara:


Ein Ayah: War is needed according to man’s present incomplete level, and it enables nations to protect or assert themselves. Normally, a nation’s success depends on its people’s bravery, and one would think that this increases with their confidence that they can succeed on their own. One would, then, think that cognitive fear of Hashem, along with a realization that "war is Hashem’s" (Shmuel I, 17:47) and that man’s own efforts without Hashem’s help are futile, would lessen the necessary confidence.
However, this is wrong, as bitachon in Hashem and reliance on His help do not come to weaken a person’s bravery. Rather, it is to teach a person that he should not rely only on his strength but should also incorporate moral strength of following the ways of Hashem. Then he is worthy of his physical strength succeeding, for then mankind has a benefit in his success. If he only thinks about his physical prowess, then success will lower his moral status, and Hashem will not want him to succeed. Thus, the need for trust in Hashem is not to weaken the human practical resolve but to make it more complete - to incorporate shleimut of the individual and the collective.
There is no clearer sign of Divine involvement in war than the falling of boulders from the sky, as it is unconnected to any human involvement. Specifically in that regard, Chazal stressed that those stones are related to the efforts (literally, on the back) of two great men, Moshe and Yehoshua. Indeed, the contributions of men like Moshe and Yehoshua were critical in the miracle, as when a person perfects himself to the point that his requests from Heaven are fulfilled, he merits miraculous intervention. These events teach us to do diligently whatever needs to be done to bring on success, whether by strengthening the nation’s resolve or improving its spiritual level, including belief in Hashem’s help.
The matter is similar to what Rav Saadia Gaon said: since we see that in spiritual matters, we make progress by our hard work to properly fulfill Torah and mitzvot, so too in physical pursuits, we will achieve success through hard work. Trust in Hashem is needed to complete the proper efforts.
The navi tells of the stones of algabish coming specifically at the time of the War of Gog and Magog, when Mashiach will come. Then Bnei Yisrael will complete their standing as a powerful, wise, and holy nation. The miracles of the war will straighten the ways of proper efforts, with the spiritual preparations complementing bravery and strength. They will not make the mistake of crediting their own power for the victory and will realize that victory comes from Hashem. They also will not make the mistake of being passive in their efforts. Rather they will be reminded of the stones Hashem employed in ancient wars, which the Rabbis attributed to the contribution of great Jewish leaders. This, after all, is the ultimate goal: that man will reach his ultimate state, both as individuals and as a collective, with hard work of the body and the soul.