Choppy, you are wrong. A milchemet reshut must be declared by a the Great Sanhedrin (but there is no need for declaration by a navi).However, there is no such need by a milchemet mitzva (Rambam Hilchot Melachim 5:2).
As for establishing a state, on the contrary, it is an absolute obligation as stated explicitly by Ramban in his Sefer HaMitzvot (and some say Rambam does not mention it because it is included in other mitzvot or basic to the Tora as he explains in the Shorashim).
Regarding the Army, it depends. If everyone is needed everyone serves (Rambam Hilchot Melachim 7:4). If not, there can be exemptions for people who are really learning (and just registered). It might even be possible to make do with a volunteer army. This is something which can only be decided by security experts -and both Rav Moshe and Rav Soloveichik said explicitly that they are the “poskim” on security matters.