The usurpation of the imperial throne (皇位簒奪)

The usurpation of the imperial throne is either when those who do not have direct blood relationship or have remote blood relationship seize the Emperor's position or when this is expressed in a critical manner. It implies that people who are not supposed to be in the Imperial Throne position force the Imperial family members to give up the position by military force or political pressure.
The usurpation of the imperial throne derives from the thought that 'the Imperial Throne is by an everlasting unbroken imperial line.'
Therefore, when the Imperial Throne shifts within the imperial line due to military power or political pressure, generally speaking, this is not an usurpation of the imperial throne. Also, basically, the Imperial Throne is said to be an unbroken imperial line and there is a controversy among scholars over whether specific examples of usurpation of the imperial throne were attempts or in fact, usurpation of the imperial throne.

Usurpation is to seize the position of the monarch. Unlike the cases of the Imperial Throne, there are many certain examples of usurpation of monarchs in foreign countries. However, even when it was practically an usurpation, sometimes, it is officially referred as a peaceful transfer of power by 'giving the position of the monarch voluntarily to someone with virtue who are not related by blood ties' which is an expression from a favorable standpoint or in praise of the method to acquire the crown from the former dynasty and it is referred as 'usurpation' from a critical standpoint. It is said that historical people who plotted it can be checked but it is unknown whether they had such intentions.

Akira IMATANI talked about the theory of the usurpation of the imperial throne by Yoshimitsu ASHIKAGA in his literary work, "Muromachi no oken (regal power of Muromacchi) usurpation of sovereignty plan by Yoshimitsu ASHIKAGA." (Chuokoron-shinsha, Inc.)
However, afraid of using the term, 'usurpation of the imperial throne' following the aftermath of Shimanaka incident, Chuo Koronsha used an inaccurate title 'usurpation of sovereignty,' overriding Imatani's opposition.

Cases of historical usurpation of the imperial throne
As mentioned above, it is controversial whether usurpation of the imperial throne did happen or it was just aimed.

Emperors in Ancient Times

There is a theory that says some Emperors in ancient times, such as Emperor Sujin, Emperor Ojin, and Emperor Keitai, usurped the imperial throne regardless of blood relations with the past Emperors. There are opinions that doubt whether Emperors in ancient times succeeded to the Imperial Throne based on blood relationships in the first place.

The Soga clan
The Soga clan prided reins of power surpassing the great king (the Imperial Family in later years) and he even tried to become the great king (Yamato sovereignty, the ancient Japan sovereignty) so, according to "Nihonshoki," (Chronicles of Japan) he was destroyed in the Isshi no hen (the Murder in the Year of Isshi).

Emperor Tenmu

Prince Otomo (Emperor Kobun) was coronated officially so there is a theory that says the Jinshin War was the usurpation of the imperial throne by Emperor Tenmu.
Going further, casting doubt over the common belief that Emperor Tenmu was the younger brother of Emperor Tenchi, there is a different view that says they were not brothers. (Katsuaki SASSA etc.)
For your information, even if this was an usurpation of the imperial throne, the blood line of Emperor Tenmu died out after Emperor Shotoku and the Imperial Family afterward returned to the Imperial line of Emperor Tenchi.

YUGE no Dokyo
While Shinto and Buddhist deities progressed to commingle and the position of the Emperor was transformed, with the entry of Emperor Shomu into priesthood (God to Buddha) and the re-enthronement of the Retired Empress Koken (Buddha to God), Dokyo, who entered the imperial court as a nursing-dhyana-priest for Empress Koken (Empress Shotoku) and had been favored by her, was enthroned as the Priest Emperor, acting correspondingly with the Emperor, and he became the de-facto co-governor with the Empress, in charge of the management Buddhism business and religious ceremonies for gods of heaven and earth. Moreover, ritual enthronement was prepared by diarchy with two people, without going through the crown prince. The Empress died shortly afterward so it was unfulfilled (a plot of Usa Hachiman-gu oracle). Although Dokyo opposed the shintaku (oracle), he was relegated to a lower position as a betto (administrator of a Buddhist temple) for Yakushi-ji Temple in the Province of Shimotsuke.

It needs to be kept in mind that "Shoku Nihongi," (Chronicle of Japan Continued) which records this incident, is a literary work from the era of Emperor Konin and Emperor Kammu, who gained the right of succession to the Imperial Throne following the Empress' death, so it is also said that it was written for the purpose of flaunting its justification.

TAIRA no Masakado

TAIRA no Masakado, after conquering Bando (old Kanto region), having won out family struggles, held a ceremony of enthronement at the local government of Kozuke in 939.
A miko, referring herself as a messenger of Hachiman Daibosatsu, made a revelation and was presented the title 'new emperor' from Okiyo-o. (Prince Okiyo)
Enthronement to the new emperor was reported to the Emperor at the Kyoto Imperial Court and they identified themselves as the new emperor as opposed to the present one. However, Masakado was busy dealing with the enemy forces and he was avenged the following year so the political aim is unclear but he established the ruling organization by assigning Bunbu Hyakkan such as Zuryo (the head of the provincial governors) in various districts in his own accord. There is a claim that says he tried to establish 'Bando independent kingdom' based on the succession of actions such as the enthronement of the new emperor as evidence.

Kanmu-Heishi (Taira clan), whose origin is TAIRA no Masakado, demoted from nobility to subject and is an Imperial descendant (posterity) which makes Masakado related to Emperor Kammu as his great-great-grandson (one theory says related as a great-great-great-grandchild).

Yoshimitsu ASHIKAGA
Due to the period of the Northern and Southern Courts (Japan), the power and authority of the Imperial Family and Court noble declined, and at the same time, the Ashikaga Shogun Family's authority advanced on that of the Imperial Family and he practically served the role of the monarch of Japan. Above all, Yoshimitsu ASHIKAGA, the third shogun enforced his influence on the Imperial Court, leading to having authority and power surpassing the Imperial Court and shogunate. On behalf of the Imperial Family, a unique diplomacy was in place which was done by receiving a sakuho (homage by Chinese emperors) as the 'King of Japan' from the Ming Dynasty in China which was to oppose the Emperor's power, the monarch in Japan. There is a theory that says this was in preparation for usurpation (another theory says that sakuho was received in order to enhance convenience of trading).

In his last years, he celebrated his biological child, Yoshitsugu ASHIKAGA's coming of age in a form corresponding with the Imperial Prince. However, immediately afterward, Yoshimitsu died suddenly without assigning the successor and the posthumous title of Daijo Hoo and tribute trade were halted by Yoshimochi ASHIKAGA, the fourth shogun and senior vassals of the bakufu, since they were historically unprecedented. There is a persistent opposing view that doubts Yoshimitsu's usurpation of the imperial throne, as the rightness of the imperial line takes place by blood line rather than from a standpoint of the Retired Emperor.

The Ashikaga shogun family is a line from Seiwa-Genji (Kawachi-Genji [Minamoto clan]) whose descendant demoted from nobility to subject so the family was an Imperial descendant (posterity) and was remotely related to the Imperial Family.

Rairen SHIMOTSUMA
Yoriyasu SHIMOMA, a boukan (a priest who served for the Monzeki families) of Ishiyama Hongan-ji Temple, publically talked about 'The concept of Shimoma bakufu' in which the chief priest of Hongan-ji Temple was to be the Emperor and the Shimotsuma clan, who hereditary succeeded the boukan of Hongan-ji Temple, was to be the Seitaishogun (commander-in-chief of the expeditionary force against the barbarians). However, there was no specific action plan to make this happen and it was not a statement by the chief priest of Hongan-ji Temple who is supposed to be the central player of usurpation of the imperial throne.

Nobunaga ODA
During the late Sengoku period (Japan), Nobunaga ODA went to the capital (Kyoto), regarding Yoshiaki ASHIKAGA as the Shogun, but shortly afterward, he faced confrontation with Yoshiaki and as a result, Yoshiaki was expelled leading to the fall of the Muromachi shogunate. Also initially, Nobunaga had been taking a cooperative policy with the Emperor at the time, Emperor Ogimachi, but as time went on, he started to act as if he was deified, even interfering with the establishment of the calendar which was set by the Imperial Count. To start with, the Imperial Count approached Nobunaga with the rank of Minister of the Right, further, presenting him with any official court rank such as the Grand Minister, chief adviser to the Emperor, and Seii taishogun (literally, "great general who subdues the barbarians") but Nogunaga declined. On the other hand, he carried out Umazoroe (a military parade) in Kyoto and threatened the Imperial Court. From such actions, there is a theory which says Nobunaga tried to become the king of Japan himself by putting an end to the Emperor. For this reason, it is said that he was killed in the Honnoji Incident by Mitsuhide AKECHI who respected the Imperial Court.
This is the so-called 'Imperial Court conspiracy theory.'

However, since remains that are thought to be a 'makeshift Imperial Palace' to invite the Emperor was discovered in Azuchi-jo Castle recently and since Nobunaga himself requested arbitration from the Emperor for the battles with Nagamasa ASAI and Yoshikage ASAKURA, and Ishiyama War, Nobunaga did not intend to put an end to the Emperor. Or it is said that it was impossible for him to do so.

However, there is a theory that says the makeshift palace was for himself.

Study of the Shinto Religion
In the study of the Shito religion, a conclusion has not been reached whether one is not qualified to become the Emperor unless one is a man with the Imperial Family blood line (unbroken imperial line) or theoretically, any powerful person with the Three Sacred Treasures of the Imperial Family can become the Emperor (note that this is not the official point of view of Shinto).