In Japan and South Korea there is deepening concern over the reliability of long-time American security guarantees – whether the U.S. will come to their aid in the event of a war. This has been turbo-charged by Donald Trump’s tough treatment of traditional U.S. allies, which has some in Tokyo and Seoul calling for a reassessment of their non-nuclear policies.
The main technological hurdle, besides enrichment, is making sure all of the explosives trigger at the exact same moment and that can be tested inexpensively without a core being present.