Japan is preparing to investigate a decades-old secret agreement that’s allowed the US to transport nuclear weapons on Japanese territory. The deal dates back to the 1960s. Successive Japanese governments have long denied reports of the pact because it violates a national law barring the storage or production of nuclear weapons. The move could further strain relations between the US and Japan. The governing Democratic Party of Japan was elected in August on a platform that included revisiting Japanese-US military ties.