to Daas2, Toi, leibedik yankel and others: Sottah mishne and gemoro 44B. Incontrovertably, milchemes mitzvah (or chovah, as per R”Jehuda), everyone goes to war. The only machlokes is whether a war to “diminish the gentiles so that they don’t attack you” is a mitzvah or not. This is the classical case of a preventive war. Otherwise, the obligation is on everyone and the Rambam paskens that way. See Rambam Melochim Perek 5 and mishne 1 and 2. Check melochim Perek 7,mishne 4.The Rambam explicitly includes a war to save jews from their enemies (melochim 5, mishne 1)as a mitzvah.
There is no “petur” for anything in such cases.
When the gemoro in sottah(44B) says “ho-osek bemitzah potur min hamitzvah” ,the gemoro means to say that the mitzvah is THE WAR and therefore people are free from other mitzvas. (see sugya and rashi). The rabbonon maintain that a preventive war is not considered “mitzvah” and therefore people are not free from other mitzvos whereas R’jehuda considers a preventive war a mitzvah and frees people from other mitzvos.
The “petur’ for people who have just gotten married, build a house,etc is only for “milchemes horeshus”- an elective war. A war to protect Jews from the enemies is a milchemes mitzvah and no one is free of his/her obligation.