Wartime

Hello Rabbi Michael,
Following your thread about the conscription of yeshiva students (and Haredim in general), I was struck by the question: What about those who deferred service in order to study for several years in a yeshiva and only then enlist (even for elite units in the IDF). From a halachic perspective of the boundaries of war, mitzvah and pikuach nefesh, is it permissible (or appropriate) to continue deferring service in order to enlist later (six months, a year, or even two years) or is it that as soon as the war breaks out, they are immediately obligated to enlist and are required to cancel their deferment of service and enlist at the earliest possible date?
Obviously, if there is an urgent need (such as was the case during the War of Independence), then there is no question at all and everyone should go out, but you mentioned in your last post the words of the Chief Justice, which are quoted as saying that even when there is no real need for more soldiers, the obligation still exists. And it does seem that this war we are in is long-term (we are talking about the order of one to three years) and a large number of soldiers will be needed in the future as well. On the other hand, it is impossible to know what a day will bring, and the situation could escalate at any given moment to the point of a real existential danger, so the issue of urgency in this matter is a bit vague.
I hope I managed to formulate the question well, I would love to hear your opinion on the subject.