The restriction, which mirrors moratoriums operating in France and Russia, has been brought about by Congressional Democrats opposed to testing, who tacked it on to a measure eagerly sought by Mr Bush, providing election year funding for the imperilled dollars 8bn (pounds 4.5bn) Supercollider atom-smashing project in his home state of Texas.
In return, and against his better judgement, the President approved the moratorium. After it expires in mid-1993, the US would be permitted 15 tests before 30 September 1996. Then, providing no nuclear power resumes testing, a full-scale ban would take effect.
For Britain, the move could have serious implications. The moratorium is purely symbolic since no tests were planned by the MoD for that period. Thereafter, Britain is allowed one test per year. But a complete ban after 1996 would run counter to John Major's insistence that as long as the UK possesses nuclear weapons, tests must continue.
Matters could be different if Bill Clinton wins the presidential election. He advocates a 'gradual' move to end nuclear testing and could seek an earlier ban.
France may find it politically difficult to resume testing when its moratorium expires this year, while Russia, is left with only Novaya Zemlya in the Arctic Ocean, where environmentalists oppose further tests. Perhaps only China will offer Washington a let-out. But even for a second Bush adminstration this could be internationally embarrassing. A President Clinton, more hostile to Peking, is less likely to allow China to point the way ahead for the US.
A resumption of testing could undercut US efforts to secure a watertight nuclear non-proliferation treaty. A UN conference on non-proliferation is due in 1995, but some non-nuclear countries say they will refrain from developing weapons only if the nuclear powers cease testing altogether.