The "grave danger" that the author is talking about is the danger of atomic weaponry
Explanation:
"Preliminary Statement of the Association of Manhattan District Scientists" was a 100-page report written after the atomic bombings in Hiroshima and Nagasaki.
The "grave danger" that the scientist is talking about in the article is the dangers of atomic weaponry. The article talks about the consequences that wrong decisions of leaders may have on its nations.
Europe felt the impact of America's slump through lack of capital, less products were being bought from Europe by Americans, American investors weren't as active, etc.
