1) why is francis marion writing to peter horry?

Actually, this post or letter illustrates the daily or day-to-day  happenings  of “Marion’s Men.”  Marion wrote this letter to Colonel Peter Horry, his subordinate, after General Gate’s defeat at Camden.  “Marion’s Men,” were not there or present at the battle since they were out destroying enemy  transportation and lines of communication , a specialty of his troops.  While working this letter, he was retreating from Camden to Lynch’s Creek in present day Kershaw Country.  The letter contains news of Marion’s recent accomplishments and includes an inventory of British troops and supplies captured.  Marion expresses a need for more ammunition, arms, horses and supplies.  While short on supplies throughout much of the war, Marion and his men effectively frustrated British actions as the Patriots struggled to hold on to South Carolina.

Explanation:

hope it helps.