That’s exactly the point. What “this” did you think was being referred to?

Yes, of course. Let me clarify:

The implication of the quoted sentence is that the U.S. dropping atomic bombs on Japan was awful and indefensible. The author then transitions to “but I want to talk about part of the Manhattan project that isn’t really related to the bombings.” What other explanation would there be for inserting this comment about U.S. history?

My comment was addressing the defensibility of the bombings. They may have been awful, but they were fully defensible. Japan was the aggressor, and all indications were that both the U.S. and Japan were going to see millions of casualties as part of an invasion of the home islands.