After Japan surrendered to Allied forces in 1945, the Potsdam Declaration required Japanese military forces to be disarmed, limited Japanese sovereignty to specific islands, and prohibited Japanese industries from rearming the country for war (among other things).
The Treaty of San Francisco (which officially declared peace between the U.S. and Japan and ended the American occupation) recognized Japan as a sovereign nation and allowed Japan to create a security force for its own defense, but reiterated that it could not arm itself to be an offensive threat.
Both the surrender and the peace treaty are pretty vague about when and how they should expire. Here we are 70+ years later, Japan is the world's third largest economy and has the world's 11th largest population. It has been a self-governing independent nation since the 1950's. And yet, it still does not have its own military. It does have the Self-Defense Forces (SDF), which is a de facto army, navy, and air force. But officially they are only a defensive force.
The Japanese Constitution contains the declaration that:
... the Japanese people forever renounce war as a sovereign right of the nation and the threat or use of force as means of settling international disputes.
...land, sea, and air forces, as well as other war potential, will never be maintained.
However, this text was directly written by the United States after the war.
Given the recent tensions with North Korea, and China's brazen claim to pretty much the entirety of the South China Sea, can the Japanese legally build an offensive military force to counter those threats? Would they be able to fight alongside other countries if (God forbid) a war broke out with North Korea or China? In other words, are they pacifists by choice, or are they still bound by their terms of surrender and the treaties they signed?