LOS ANGELES — Forty years ago this month, more than 50 nations gathered in the East Room of the White House to sign the Treaty on the Nonproliferation of Nuclear Weapons (NPT). In his memoirs, U.S. President Lyndon B. Johnson called it "the most significant step we had yet taken to reduce the possibility of nuclear war."

Today, with the benefit of time, we can evaluate whether the accord truly marks the "historic turning point" Johnson hoped for. The evidence suggests that while the pact's dikes have largely held, serious leaks have developed, prompting nuclear vigilantes to apply force when they have concluded that diplomacy would fail to halt the nuclear bomb's spread. Whether this behavior is a harbinger for the future remains unclear, but it raises a continuing specter given the failure of the NPT to include an effective enforcement mechanism.

One fact is not in doubt: the NPT is the legal linchpin for the nuclear nonproliferation regime now signed and ratified by all but three nations — India, Pakistan, and Israel — and one dropout, North Korea.