The First Shogi Pieces are Moved
Fall of Yokosuka
2nd of Tsukigatsu, 3578 (22nd of January, 1144)
The Fall of Yokosuka was without a fight to Yamashita marked a major turning point in not only how Yamashita's own rebellion faired, but ultimately could be argued was the event that triggered the start Heishin War formally for a number of reasons.
First, Yokosuka was the second largest port in the east, and was home to a large part of the Eastern Fleet. Said Fleet, like its Western counterpart thought themselves to be above the mere squabbles of the interior. But with a Daimyo taking over this port without a fight was quite shocking to the Navy. They needed to figure out fast how to deal with this situation because suddenly, those petty squabbles had now officially become a concern of the Navy.
Secondly, Yokosuka's fall sent shockwaves across the Empire because it was a clear escalation from Yamashita in comparison to the past. Originally, the purpose of his rebellion was to save his own neck from the vengeful wrath of the Shogun of the East, Hiroyoshi, and he had just fought the local daimyos to stay in power. There wasn't a question of his clan losing all of its power, but more of him being removed and likely killed. Now that he took the capital of the Wakabayashi Domain, things drastically changed.
In his "Yokosuka Declaration", he declared that he, Iesada Yamashita, no longer bowed to the word of the Shogun but to the Emperor alone like the Imperial Navy did, and furthermore that the Shogun's power was illegitimate because of how it had been abused. He called on the Emperor to revoke their status immediately, and rule without such petty tyrants. He also called on other Daimyos, to join him, for he believed that only the rule of the Emperor would bring stability to not only himself, but all domains and all peoples.
Edo's Reaction
29th of Tsukigatsu, 3578 (19th of February, 1144)
When word reached Edo a month later, Shogun Harumitsu Mononobe was infuriated by this defiance. He personally took offense towards the statement that he ruled without legitimacy, as the Emperor had granted him and his family that many generations ago to rule on his behalf. Furthermore, his positioning of his "right" being equal to that of the Navy was nonsensical and given that the Commander of the Yokosuka Naval District had been all too willing to overlook this rebellion, he firmly believed the Navy was no longer to be trusted either.
Given that many of his vassals were no longer trustworthy, he decided to do what would prove to be a grave mistake. He issued an "Edict of Loyalty", that demanded that each and every domain under his rule contribute to the campaign against Yamashita he was going to undertake, and that those that refused would have their lands confiscated. Furthermore, he issued a bounty on the head of Yamashita himself, worth a small fortune to whomever would claim it and sent a letter of protest to the Imperial Naval Admiralty regarding the events at Yokosuka, demanding that the commander who just let a rebel take over the port which by every right should have been defended by the Imperial Navy be brought before a military tribunal.
The latter would cause a great division in the Imperial Navy itself, but that is another story. What became apparent right away from the orders issued from Edo was that Harumitsu was done playing around with the Daimyos and it was either loyalty, or death. If the Battle of the Takase River, and other affairs emboldened them to potentially unite, these demands however just legally they would be the true first shogi piece moved on the board. There was no going back now, and what could have been easily resolved through negotiations would now grow into the full-blown revolt of several powerful Daimyos and great division within the Imperial Navy itself.