Complex sentence: The 3rd Regiment of Artillery was mustered into federal service between 15 September 1812 and 15 December 1812 and again between 2 September 1814 and 3 December 1814, and its batteries assigned to various forts in and around New York City and on the islands in the harbor.
Split into simpler sentences: The 3rd Regiment of Artillery was mustered into federal service between 15 September 1812 and 15 December 1812 and again between 2 September 1814 and 3 December 1814. The batteries of the 3rd Regiment of Artillery were assigned to various forts in and around New York City and on the islands in the harbor.