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.
Eh, you would still want to test. Even buying from an ally, there’s no telling if the blueprints haven’t been sabotaged to be ineffective or maybe just inefficient. (Maybe your ally supports you having nukes, but wants to make sure their nukes are better.)
But even beyond that, when you test a nuke, you’re not just testing the design. You’re testing your materials. You’re testing your manufacturing capability. You’re testing every bit of the vast production process that went into making the weapon. And you’re testing your own technical ability to design nuclear weapons. Getting blueprints would be beneficial, but there’s no real substitute to designing your own bomb optimized to your own available materials and production processes.