Firstly, one should make sure Mark doesn't carry any modern bug or virus with him. That would most likely wreck havoc on most of France, if not the entire continent.

That aside; Who says time needs to be linear necessarily? The specific timeline that Mark enters might just as well (in that instant) become an alternative/temporally transposed universe. Simply because he went there. In this case, apart from considerations on his own health and security, Mark will experience a veritable free-for-all fun zone during his stay. Nothing he does will have any effect on the present-time-universe that he hails from. (Insert appropriate moral-ethical discussion here). He won't be able to read about his exploits in the historybooks, though. At least not the historybooks from his own present-time-universe.

Firstly, one should make sure Mark doesn't carry any modern bug or virus with him. That would most likely wreck havoc on most of France, if not the entire continent.

That aside; Who says time necessarily needs to be linear? The specific timeline that Mark enters might just as well (in that instant) become an alternative/temporally transposed universe, simply because he went there. In this case, apart from considerations on his own health and security, Mark will experience a veritable free-for-all fun zone during his stay. Nothing he does will have any effect on the present-time-universe that he hails from. (Insert appropriate moral-ethical discussion here). He won't be able to read about his exploits in the history books, at least not in the history books from his own present-time-universe.