What does insure domestic tranquility mean in the preamble

How is the city of savannah related to the union strategy of a full blockade of confederate ports? a) savannah was the last port needed to be secured in order to totally blockade the coast of georgia. b) savannah was the least significant city in georgia, and the union strategy centered on securing atlanta and capturing the tennessee river. c) savannah was the least significant city in georgia, and the union strategy centered on securing atlanta and capturing the chattahoochee river. d) savannah was the most significant port in georgia, and the union strategy centered on securing bases at savannah and the outlying islands to fully blockade georgia.