Why assume that the game would be set during the first contact war, could be Mindoir, that was a major conflict which was followed up with a lot of counter attacks I think?
Seem to recall something in ME/ME 2's codex about the alliance relieving Mindoir and then going on to curb stomp batarians/pirates as a 'warning' .
Hell it could be any number of scenarios since the first game makes it clear than Alliance Marine units get sent to a lot of places for 'recon' and 'other' missions, for example the first time you run into a thresher maw*. Hell, doesn't N7 mean that shep was part of special forces, they must do something beside practice bad ass one liner interrupts and reporter punching.
/edit personally if we are doing spin off games, can I has spaceship commanders plz? Just so I can say "Sir Issac Newton is the deadliest son-of-a-***** in space" A lot and not seem too odd.
*Also screw you bioware and the horse you rode in on, obvious traps are fine but god damn I was always scared to drive the Mako out onto flat open ground anywhere, cuz it was thresher country. probably added about 20 hours to my game time cautiously advancing
