> Telling Bnei Torah to leave the Yeshiva and learn less so they can enlist in the army, means less Torah and less protection for Klal Yisroel.
This is mentioned in Maseches Megila. Malach threatens Yehoshua for neglecting karbonos during the day and learning Torah during the night (due to army service) – and then clarifies that a death might come for neglecting learning. Note that there is no suggestion to abandon the military campaign and not even learning during the day. So, as we have no karbonos right now, serving during the day and learning at night would be acceptable by Yehoshua’s standards.