Imagine that you are Shinzo Abe. The cause that drives you, above everything else, is the revision of the constitution that was imposed on Japan by the Americans after World War II. There are a number of reasons for that: part of it is the desire to “normalize” your country and its defense policies; part of it is a denial that Japanese aggression caused the war (understandably called the “War of Japanese Aggression” in China); but a big part of it is a realistic appraisal of the military and economic threat that a rising China presents to an island nation with long, exposed sea lanes.
The current constitution has substantial support in the Japanese public and will be difficult to change. Fortunately for you, you have two unlikely allies: Kim Jong-Un and Donald Trump. The former has been lobbing missiles through your country’s air space, and needs to be deterred; the latter complains endlessly about America’s trade deficit with Japan, and can’t be relied upon in a conflict with China or North Korea.
There is a logical way to deal with this situation: buy hundreds of billions of dollars worth of arms from the Americans, including nuclear weapons, in order to close the trade deficit, protect the nation from the crazoid in North Korea, and free yourself from dependence on the American military. That approach solves all your problems at once, and provides a strong justification for your constitutional amendment.
Now, imagine that you are Xi Jinping. You have always maintained that, however bad the regime in North Korea might be, letting it collapse would be worse. Is that really true if maintaining the regime means that Japan will have nuclear weapons?
There are no good options for Xi here.