No nation in the second ward war has enough merchant shipping. You've got to go and make some, and that takes years. By late 19 forty four, american submarines are doing to japan what german u boats hoping to do to britain,. back during the battle of the atlantic in 19 forty two starve an island nation of the resources it needs to continue to fight the war. The only way that that number will slow down is because eventually they'll run out of targets.