Answer: The phrase "The East Asian constraints on a transfer thus slackened" likely refers to the easing or relaxing of restrictions or limitations on the movement or transfer of goods, resources, or people in the East Asian region. In this context, "constraints" refers to factors that limit or restrict the transfer of something, while "slackened" means that these limitations have been reduced or become less stringent. The phrase suggests that the restrictions on transfer have become less restrictive or less strict, allowing for a greater degree of movement or transfer in the East Asian region.
Explanation: The meaning of "The East Asian constraints on a transfer thus slackened" in this context is that the limitations or restrictions on the movement of a portion of the fleet from Pearl Harbor to the East Asian region were becoming less strict or stringent. This suggests that the situation in the region had changed in such a way that removing a portion of the fleet was becoming less likely to lead to a Japanese advance or have a significant impact on bargaining. The constraints or limitations on the transfer had become less restrictive, making it easier for the United States to consider removing a portion of the fleet without worrying about potential consequences.