Complex sentence: The fort was declared a National Monument in 1924 and after 251 years of continuous military possession, the fort was deactivated in 1933 and the site was turned over to the United States National Park Service.
Split into simpler sentences: The fort was declared a National Monument in 1924, and after 251 years of continuous military possession, was deactivated in 1933. The site was then turned over to the United States National Park Service.