Complex sentence: In April 1861 while his ship was in New Orleans, he offered to assist the Confederate States of America, when he returned to New York, he found a letter from Confederate Attorney General Judah P. Benjamin accepting his offer and ordering him to for his assignment.
Split into simpler sentences: In April 1861 while his ship was in New Orleans, he offered to assist the Confederate States of America. When he returned to New York, he found a letter from C.S.A. Attorney General Judah P. Benjamin accepting his offer and ordering him to, for his assignment.