Complex sentence: During the 1940s the U.S. Navy purchased about 4/5 parts of Vieques grounds by compensated land expropiation, and then later started using them as target practice for bombs and missiles, and for other ground practices--like the USMC beach and helicopter infiltration exercises.
Split into simpler sentences: During the 1940s the U.S. Navy purchased about 4/5 parts of Vieques grounds by compensated land expropiation. Later, the U.S. military later started using them as target practice for bombs and missiles, and for other ground practices--like the USMC beach and helicopter infiltration exercises.