Complex sentence: He was busily employed up to the end of his life in writing treatises, pamphlets and open letters on subjects of military art and history, and in 1859 he was asked by Napoleon III to furnish a plan of campaign in the Italian War.
Split into simpler sentences: He was busily employed up to the end of his life in writing treatises, pamphlets and open letters on subjects of military art and history. In 1859 he was asked by Napoleon III to furnish a plan of campaign for the Italian War.