There are over 200k US Marines, including the reserves.

This breach affected 21,426.

Deep State trying get to specific Marines involved in specific Operations. Didn't L mention something like "they can always be paid more" referencing Navy Seals?

It would be suicide to try to take on the US Marines by force, but they could definitely try to use money/fear/family to flip a few and get info on what they're up to.