Inspired by this post. Apparently marriage of a Japanese royal to a commoner is morganatic, removing them from the line of succession. But who in Japan is a commoner? Or rather, who in Japan is sufficiently noble to not make marriage morganatic? Would marriage to foreign royalty or nobility be morganatic? I note Tonga, for example, has a prince who is single, Prince Ata.
It’s my understanding that only Japanese royalty count. Note the males can marry commoners, and retain their status, but females cannot. The current Emperor and the recently retired Emperor married commoners, as well as the Crown Prince. Actually, now Japan now longer has aristocracy outside of the Imperial Family.
At the end of WWII, United States was concerned that an outsider could marry into the Imperial Family and gain power, so they pushed the Japanese to reduce the line of succession to the descendants of Emperor Taisho, father of Emperor Hirohito (Emperor Showa). There had been a rather large system of various princes, counts, barons, and such, and this was eliminated as well. Many had been involved in the war, including some generals, as well as a prime minister shortly before the war.
The decision was made to allow the emperor to retain the throne, but to strip him of all political power, and to eliminate the aristocracy, another source of potential power.
I never understood why they thought that an “outsider” would be more of a concern. Why would they assume that the grandson of the emperor inheriting the throne wouldn’t be a problem, but the grandson of his cousin was likely to be one?
According to my left elbow, they were actually thinking “not-American foreigner” rather than what actually ended up being implemented. “Suspicion that the royal policies might be influenced by the nationality of origin of the royal spouse” is quite a recurring theme in history.
Has the royal family ever married non-japanese?
I’m no expert but I think there were Korean/Japanese marriages.
i’m not sure why this has its own thread, but in the other thread I talked about how the Emperor and his family live in gilded cages. Obviously the more control there is over a family, the less likely that someone would act out.
Emperor Taisho was the only son of Emperor Meiji to survive to adulthood. He had sisters who had children but they would not have contributed to the line of succession as it’s strictly through male linage.
It’s actually worse than that. You have to go back to the 1600s to find any emperor who fathered more than one son who survived until adulthood.
So it wasn’t a first cousin that they were worried about but a ninth (or so).
There were several cadet branches which went back many hundreds of years and which were removed.
This can’t really looked at from the modern perspective of a quiet and peaceful Japan but has to be examined from the historical circumstances.
There were many, many people among the Allies who wanted to eliminate the position of Emperor. Placing strict controls one method of reassuring these people that Japan wouldn’t repeat the same history.
No. You need to stop listening to your left elbow. It flunked history. 
Japan has not had the same history of foreigners interfering with the imperial family. Europe swapped royals around but the Japanese have only had Japanese.
Except that they’re Koreans. The Imperial family started with the Yamato family in the sixth century. They were descendants of Koreans from the Paekche Kingdom. People don’t like to talk about that, though.
It wasn’t the Japanese who were afraid of outside interference, it was the Americans. You of all people should be conscious that those two happen to be different countries 
and of Americans’ bias regarding world history (every country has its biases, of course, but the American bias is certainly a lot more UK-centric than Japan-centric).
No. The Americans were not concerned at that at all. The Americans knew that the Japanese Imperial Family would never accept foreigners into the family.
The Americans had just fought the bloodiest war in history and was in no mood to repeat it.
There was never a concern by the Americans or the Japanese about foreign influence on the Imperial Family.
Period.
Isn’t it also anyone’s guess who fathered Taisho’s son? Hint, it probably wasn’t Taisho…
I still don’t get it. Why would the ninth cousin, or the member of a cadet branch, more likely to be a problem than the son or grandson?
It’s about genetic purity.
So, let’s say a Japanese prince married Princess Charlotte, would that be morganatic? How about a Saudi princess?
Because the son and grandson are raised in guilded cages so they have been indoctrinated from childhood. They have been educated within the Imperial System. They go to elite schools, but they are raised with the expectations of some day becoming the emperor or a member of the Imperial Family so the are constantly being molded into the expected person.
Offspring from cadet branches are outside of the control of the Imperial Household.
As was shown the other thread, the Imperial Household Agency actually controls a lot and harassed the retired emperor’s wife, especially when she newly married to the Crown Prince.
The current Empress was harassed as well in the past, and the then Crown Prince made a public complaint. An outsider would have likely been much more vocal or taken other steps. All conjuncture on my part, of course.
Now in 2019, after almost 75 years of peaceful existence we are used to quiet Japanese royalty, but at the end of WWII, it would not have been as easy to see that.
Did you read the thread?
From a broad historic perspective, offspring from cadet branches offers a competing quasi-legitimate focus for dissenting groups to rally around. Think birthers: you decide that the current emperor isn’t legitimate for whatever reason, which means this other guy is, and you use that claim to legitimize a rebellion/coup. It doesn’t matter if the actual descendant is the instigator: he may be almost a prisoner of the actual agents. All that matter is that as long as he exists, he provides a potential competing narrative.
This is why Ottoman emperors had all their brothers killed, down to toddlers; they were tools, even before they could talk.
Yes I have; I’ve obviously missed the answer.
nm
Post 2, the first answer to your OP. It directly answers your question.
It’s not, because the cadet families have the same genetic purity. They are also from the same male linage, just from lower ranked sons.
This is directly connected to Japan and drama at the end of WWII. For a quick summary, by late spring of 1945, Japan had been almost completely destroyed, with the US having completed the invasion of Okinawa and almost all of her large and even medium cities having been burn down in the great fire bombings. However, the home island themselves had yet to be invaded. (Okinawa was part of Japan, but not on the same status.)
Japan was trying to find a way to surrender with terms, while the Allies were insisting on an unconditional surrender. It all came to a head in August when both the atomic bombs were dropped and the Soviet Union joined the war against Japan.
The Emperor is said to have tempered his responses to the situation out of fear that the Army would do exactly that, depose the Emperor and find a replacement more aligned with their hardcore attitude. There actually was an attempted, but failed coup after the Emperor had announced the surrender.
That all said, I haven’t read anything going into the details of the decision-making process or the reasoning for it. They simply do not discuss this much.