lebidik yankel:I suggest you re-read the sugya. What the rabbonon do NOT consider a mitzvah but R”jehuda does is a war to PREVENT gentiles from waging war upon Jews. THAT IS THE PIRUSH of “lime-utei kutim delo leisi aleihu” . Then look in the rambam hilchos melochim where he EXPLICITLY says that a war to save jews from their enemies is a milchemes mitzvah LEKULEI ALMA. (seee Lechem Mishneh). He puts it in the same bracket as milchemes shiva ammemim that is certainly a milchemes mitzvah.
Your pirush of “osek bemitzvah” is erroneous. THE WAR is the mitzvah that pre-empts the obligation of other mitzvas-according to R’jehuda and not according to the rabonnon. It is a preemptive war, as the geomor says, not when the war is alrady happening. In such case everyone is mechuyov-as per gemoro and rambam.