Complex sentence: During this brief war, one Pony Express mailing, which left San Francisco on July 21,1860, did not immediately reach its destination; the mail pouch (Mochila) did not reach St. Joseph and then New York until almost two years later.
Split into simpler sentences: During this brief war, one Pony Express mailing, which left San Francisco on July 21,1860, did not immediately reach its destination. That mail pouch (mochila) did not reach St. Joseph and subsequently New York until almost two years later.