Final answer:
Admiral Leahy's opinion of the atomic bomb itself is succinctly captured in the phrase "barbarous weapon," signifying his disapproval of its use against Japan.
Step-by-step explanation:
The phrase from the passage that gives Admiral William Leahy's opinion of the atomic bomb itself is "barbarous weapon". Leahy describes the atomic bomb as a brutal and unsophisticated tool of war, akin to the practices of the Dark Ages. This phrase clearly conveys his negative stance on the morality of using the atomic bomb on the Japanese cities of Hiroshima and Nagasaki, highlighting his belief that it was unnecessary and that Japan was seeking surrender.