Step-by-step explanation:
The phrase "his only fully developed exposition in 1941 of the strategic threat" means that Roosevelt's May 27 speech was the most comprehensive and detailed explanation he gave during that year about the threat facing the United States and his plans to address it. The speech was an important statement of policy, reflecting Roosevelt's understanding of the problem and the steps being taken to protect American interests in the Atlantic. Although he did not refer to specific cases, his speech made clear that the chosen course of action carried the risk of war, and he warned the American people accordingly. The speech was well-received by the press and public, and it helped to increase Roosevelt's popularity during that period.