Question

Hello Rabbi Michael,
Following your column about drafting yeshiva students (and Haredim in general), a question occurred to me: what about those who received a service deferment in order to study for several years in a yeshiva and only then enlist (even in elite IDF units)? From a halakhic standpoint, in terms of the parameters of an obligatory war and saving life, is it permitted (or appropriate) to continue the deferment in order to enlist later on (half a year, a year, or even two years later), or once the war broke out does an immediate obligation take effect for them to enlist, such that they are required to cancel their deferment and enlist at the earliest possible date?
It is clear that if there is an urgent need (like there was in the War of Independence), then there is no question at all and everyone must go out [to fight]. But in your last column you mentioned the words of the Chazon Ish, who is cited as having ruled that even when there is not actually a need for more soldiers, the obligation still exists. And indeed, it seems that this war we are in is a long-term one (people are talking on the order of one to three years), and there will be a need for a large number of soldiers later as well. On the other hand, no one knows what tomorrow will bring, and the situation could worsen at any moment to the point of a real existential danger, so the issue of urgency here is a bit unclear.
I hope I managed to phrase the question well. I would be glad to hear your view on the matter.