Answer: "The undercurrent of concern showing, that he was sure the British would not allow any great debacle or surrender" means that the speaker noticed an unspoken or implicit worry or anxiety in someone's behavior or tone of voice. The person being referred to was confident that the British would not allow a significant failure or surrender to occur, but the speaker detected a subtle expression of concern that indicated the person was not entirely convinced or felt uneasy about the situation. The phrase "undercurrent of concern" suggests that the person's anxiety was not overt or expressed directly, but rather implied or hinted at.
Explanation: The phrase "the undercurrent of concern showing, that he was sure the British would not allow any great debacle or surrender" means that while President Roosevelt acknowledged the seriousness of the situation in the Middle East and the impact it could have on British morale, he also believed that the British would not suffer a major defeat or give up entirely.
The phrase "undercurrent of concern" suggests that there was an unspoken or implicit worry among some individuals about the British ability to withstand the pressure of the war. However, Roosevelt was confident that the British would not allow a "great debacle or surrender," meaning a significant military defeat or complete surrender to the enemy.
Overall, Roosevelt believed that the British would ultimately prevail in the war, and he was concerned about the impact of British morale on other neutral nations and subjugated peoples. He believed that control of the Indian Ocean and Atlantic would ultimately determine the outcome of the war.