The Northern Mariana Islands were not a permanent legal possession
of Japan at the time of the war as it had only been entrusted to Japan
under a mandate by a group of countries through their organization -
the League of Nations. Therefore, the United States could not strip
territory from defeated Japan at the conclusion of the hostilities since
the islands were never recognized as permanent legal possession of
Japan in the first place. On July 1947 the area was recognized as a
Trust Territory by the United Nations.
The United States Navy, and later the Department of Interior, became
the administrator under a Trusteeship Agreement with the United
Nations Organization, the successor to the League of Nations.
In 1952, upon signing the Treaty of Peace in San Francisco, Japan
legally gave up all claims in the mandated islands formerly provided
by the League of Nations and acknowledged the United Nations
Agreement establishing the Trust Territory Of The Pacific Islands with
the United States as the administering authority.
The reconstruction of the economy of Saipan after the war was long
in occurring with the result that the area was the last of the former
battlefields of World war Two to recover from the devastation.
This process did not really start until around 1978 some 33 years
after the termination of World War two.