system
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human
[Mass Effect] Why was Shepard (a marine corps officer) made the XO on a navy ship?

Shepard is part of the Alliance Marine Corps and the N7 special operations force, trained in all matters of infiltration, ground combat, sabotage, etc. Why were they assigned as the Executive Officer of a navy vessel? They don't have the skills or training required for command of a star ship.
gpt
The Alliance doesn't have a Marine Corps like the larger wet navies back on Earth. Alliance Marines are actually a sub-branch of the navy, with individual ground units treated much the same way as fighter squadrons. Although they have their own rank titles, that's more a matter of tradition than anything else and every marine is considered naval personnel and trained as such.

Kaiden Alenko, for example, served as part of the marine detachment of the original SSV Normandy but also worked in the cockpit during routine ship operations.

In addition, the ICT system provides training in leadership skills as well as combat. Couple that with Shepard's own naval training and natural leadership and tactical ability, and it's not too much of a stretch for Shepard to be given command of a ship. Especially with the endorsement of the Citadel Council and the urgency of the mission to stop Saren.