They told me in Marine Corps boot camp that that would be a war crime

This is a clearly defined war crime under the Geneva conventions

"Medical Units [..] lose their protection if they are being used, outside their humanitarian function, to commit acts harmful to the enemy." (Rule 28. Medical Units)

https://ihl-databases.icrc.org/en/customary-ihl/v1/rule28

@yard Treating wounded fighters does not meet that definition.

@BTE Oh, my mistake. I thought it was an actualic question, not a theoretical one

I suppose it depends what "attack" means. If you mean bomb the place and kill everyone inside (including all of the doctors and nurses and support staff), then this should obviously be NO (that's a clear violation of the Geneva convention). However, if you mean capture the troops inside without killing anybody, then probably YES. There are gradations between these two extremes, but I think in the normal usage "attack" means something closer to the first.