The U.S. government weighed its chances of convincing Tokyo in the late 1960s to allow the deployment of nuclear weapons on mainland Japan if an East Asia crisis broke out, declassified documents showed Sunday.

The idea, which was never proposed because it was apparently considered to have a "very slight" chance of being accepted, offers a look into how Washington sought to expand its military footprint in the region after World War II and the Korean War.

The atomic bombings of Hiroshima and Nagasaki had left the Japanese public with a strong aversion to nuclear weapons, with the prohibition of the possession, manufacture and introduction into Japanese territory of the weapons -- first outlined in 1967 -- coming to form the core of the country's nuclear policy.

The documents, dated June 26, 1969, are comprised of drafts of a joint communique by the then-president Richard Nixon and the prime minister of Japan at the time, Eisaku Sato. The documents outline the role of the U.S. military after the return of Okinawa to Japanese administration in 1972.

One of the drafts notes that if the two countries agreed that a "state of emergency existed in East Asia threatening imminent armed attack" on Japan, steps would be taken to "enable the U.S. forces in Japan to introduce the necessary forces and equipment to meet the danger."

While the two countries eventually made a secret agreement allowing the introduction of nuclear weapons to the southern islands even after the handover, the documents reveal for the first time the U.S. government's desire to locate part of its nuclear arsenal on the mainland, according to Masaaki Gabe, a professor at the University of the Ryukyus in Okinawa.

The draft, written by the U.S. State Department, is labeled as "including all of the points desired by the U.S....whose chance of full acceptance by the GOJ (government of Japan) is very slight."

Gabe obtained the previously top secret documents from the U.S. National Archives.

The documents also include a separate draft of the joint communique that the State Department saw as "possibly more acceptable" to Japan.

That draft included a secret agreement that should a crisis arise in the region, the Japanese foreign minister will promise to give "prompt and sympathetic consideration" to consent to requests to deploy the U.S. military on the mainland via the U.S. ambassador to Japan.

Under the agreement, U.S. forces would also be allowed to conduct military operations out of Japan if an emergency arose on the Korean Peninsula or in Taiwan without prior consultation with Tokyo, which it is obligated to do under the Japan-U.S. security treaty.