Imagine this scenario: Imagine that Hitler and Stalin both died in, say, 1935 and replaced by unaggressive Euroweenies of the Gorbachev sort. Without Hitler's bad example, the leaders of the Japanese Navy arrest the leaders of the Japanese Army and Japan gives up the crazy idea of fighting America. Or imagine whatever you want (Edwin Starr Sr. records "War! What Is It Good For?" in 1938, if you like), as long as it leads to WWII and the Cold War never happening. By 1939, Europe and East Asia have already settled into their long post 1989 peace that we know from our world.

In that situation, would anybody have gotten around to inventing the atomic bomb? Who? When? (Something to consider is the question of whether, in this world, nuclear weapons were ever truly reinvented?)

Now, imagine another scenario in which it's 2010, but nuclear technology has remained at rudimentary 1930s levels ever since the 1930s. Everything else is the same about the world of 2010 — They've got computers, lasers, titanium, Powerpoint, Twitter, whatever — just that nobody ever got around to working on nukes. 

Then, some country in this Alternative Present decides to build an atomic bomb. With all the 21st Century advantages, would they do it faster than the Manhattan Project? Would they even set one off by 2020?