Complex sentence: In February 1942, the Gulf Sea Frontier was established to help guard the waters around Florida, and by June of that year, more attacks forced military leaders in Washington D.C. to increase the numbers of ships and men of the army group.
Split into simpler sentences: In February 1942, the Gulf Sea Frontier was established to help guard the waters around Florida. By June of that year, more attacks forced military leaders in Washington D.C. to increase the numbers of ships and men of the army group.