From State Department Correspondent Andrea Koppel

WASHINGTON (CNN) -- When India and Pakistan exploded a series of nuclear devices last month, it was an anything-but-polite knock on the nuclear clubhouse door.

The five countries already officially inside that club -- the United States, Russia, China, Britain and France -- made it clear that India and Pakistan were not, and would not be, welcome.

But the obvious question is, why not? Why should other countries be denied admission to the nuclear club?

The answer: timing. The five existing nuclear powers had all tested their nuclear devices by the early 1960s, when a movement began to stop the spread of nuclear weapons. In a policy that some critics call discriminatory, those five countries then crafted a treaty that said, in effect, the nuclear club is closed.

Double standard? - CNN's Andrea Koppel reports that timing is the key to membership in the nuclear club

Under terms of a non-proliferation treaty signed in 1968, the members of the nuclear club struck a deal with those on the outside.

"The haves wouldn't spread (nuclear technology) and would reduce their arsenals. And the have-nots wouldn't acquire it," said Joseph Cirincione of the Carnegie Endowment for International Peace. "And for years, there's been this argument from the have-nots, pointing to the haves saying, 'Why aren't you living up to your part of the bargain?'"

Last month's tests by India and Pakistan were the first real challenge in decades to the existing nuclear powers. But there is no easy answer as to how those five countries can respond to the challenge.

Critics say to welcome India and Pakistan into the club as official nuclear powers would encourage other countries to follow their lead and, in essence, shoot their way inside. On the other hand, to officially exclude them would mean that they would be outside of the non-proliferation constraints practiced by the five traditional nuclear powers.

And that could leave the nuclear club with no legal framework to stop the spread of nuclear weapons capability to other nations.