Bottom line
You cannot be sent to active duty on land overseas until you have finished your basic military training. For a Marine still in the training pipeline, that means recruit training and SOI/MCT (enlisted) or OCS and TBS (officers), plus MOS school, before you can be activated to a mobilized unit. This is short. It is one rule about deployability, and it is the gate every Marine has to clear before going overseas.
What it means for you
If you have already completed all of your initial training and your MOS school, this rule is already behind you and there is nothing to do. If you are a non-prior-service Marine in the Selected Reserve still going through Initial Active Duty for Training (IADT), or you are in the Initial Active Duty for Training (IIADT) Program and have not finished, you will not be activated to a mobilized unit until your entire IADT requirement is done. The point is simple: finish the pipeline first, then you are deployable.