Answer: The meaning of "American battleships operating under the constraints of neutrality in the Atlantic were no substitute for American battleships deterring Japan in the Pacific" is that the presence of American battleships in the Atlantic, which were restricted by the rules of neutrality, was not an adequate replacement for the presence of American battleships in the Pacific, which would have deterred Japan from taking aggressive actions.
In this context, "neutrality" refers to the policy of not taking sides in a conflict and not engaging in any hostile acts. American battleships operating in the Atlantic were limited by the rules of neutrality, meaning they could not take any actions that would be seen as hostile towards the nations involved in the conflict.
However, having American battleships in the Pacific, which would serve as a deterrent to Japan, was considered more important. The presence of American battleships in the Pacific would signal to Japan that the United States was prepared to defend its interests and would serve as a deterrent to any aggressive actions. The idea is that the mere presence of American battleships in the Pacific would prevent Japan from taking any hostile actions, making them an effective tool for maintaining peace and stability in the region.
Explanation: The meaning of "American battleships operating under the constraints of neutrality in the Atlantic were no substitute for American battleships deterring Japan in the Pacific" is that, according to Churchill, while any movement of American battleships into the Atlantic was appreciated, the Royal Navy believed that having American battleships in the Atlantic, under the restrictions imposed by neutrality, was not an adequate replacement for having American battleships in the Pacific, serving as a deterrent against Japan. In other words, the presence of American battleships in the Atlantic was not seen as sufficient in deterring Japan and ensuring the security of the Pacific region.