Maybe we could buy back the information from the Russians? Kidding. Did we really lose the ability to make tactical nukes?
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Not tactical per se, but the guy who was the genius designer quit (cancellation of Orion was one reason) and no one replaced him. The nukes for Orion were much smaller size, yield, and of a different design than those for warfare using tactical warheads. You cannot replace one with the other.
The Russians are also in the dark - thus the art was lost.
All current new design is theoretical - no actual tests, without which no one knows if a design will really work.
One can make modest changes to old designs known to work - for instance, but something totally new may not until one is exploded.
There is rocket science and then there is nuclear bomb design. Neither of which the USA spends much intellectual capital, political will, money or effort on any longer. There are just too many homeless to feed, you know.