Sentence 1: The Americans now have a base to attack Japan from. 
 Sentence 3: From there they submarines can reach Japan's home waters 
 Sentence 4:  Five or six submarines go out on patrol at a time 
 Sentence 5:  Every month they manage to sink over 100,000 tons of enemy shipping 
 Option 1: They've staged submarines at Midway Island. 
 Option 2: They still have no submarines however.