Prior 593 ("Pre Buddhism")   While much of Japan's early history was fabricated, it is easy to make too much of these lies.  Japan is not the first nor last culture to find it convenient to lie about the past.  Heck, these days counties even lie about the present! Also, as many an individual has found out for themselves, when so much of what has been said is untrue, how and when does one start telling then truth.   Anyway, the important point is that in 593 AD, Japan was still just obscure group of island relatively far from the "middle nation" - China - and significantly less populated.  Japan was not the end of the earth, but you could see if from there. China on the other hand laid claim to the world largest city Xi'an, the capital of the Sui Emperor, whom after after nearly four centuries of political fragmentation, unified China in 589 AD.   And while the Chinese and the peoples of the Korean peninsula had been fighting for at least a thousand of years prior, the Japanese culture was really only "getting rolling" around 200 to 300 AD after the import of "modern agricultural" and "metallurgy" technique from China and Korea that resulted in explosive population growth in and around "central Japan" - Kansai   This may comes as a surprise to many whom actually think Japan is an ancient culture - not true.  My thoughts on this period are currently being rewritten and will be available again soon.  While not directly involved (at all), two important wars were occurring in the region towards the end of this period.  First was the war of the Baekje and Silla (peoples the Southern Korean Peninsula) against the Goguryeo, the very powerful and successful empire occupying northern Korean peninsula and Manchuria.  At his time the Baekje royal court and king (a practicing Buddhist) was "very good friends" with Soga family, one of the three most powerful of the Japanese imperial court families and a family whose power within the imperial court was rising at a fast rate.     The second was the Wei Chinese war against the Goguryeo starting in 598 AD.   Japan was far away from all this activity but it is important to understand the context in which local events were occurring.   Anyway, I choose 593 as the end of this period because it coincides with the resolution of a major 5 year power struggle within the imperial court that coincided with the the assassination of Emperor Sujun (32nd soverign), the "ascension / appointment" of Empress Suiko (the highly qualified but compromise candidate), the naming of Shōtoku Taishi as crown prince and regent, and other important changes that began thereafter.  Shōtoku Taishi  was pretty cool dude who was purportedly able to have 12 separate conversation simultaneously with 12 people and who is commonly credited with promoting Buddhism throughout Japan.  While much that is said about him is probably not true, what is true is that as regent, Shōtoku Taishi controlled the 33rd imperial court of Empress Suiko and implemented four very China oriented policies.  First, was the official recognition of Buddhism as a national religion in 594 AD with the "Flourishing Three Treasures Edict", second was the beginning diplomatic relations with the Sui Emperor of China.  Third, in the year 603 AD,  the imperial court adopted the same 12 level "merit based ranking system" use in China for classifying it's members.  and fourth, the adoption in 604 AD of a seventeen-article "constitution" which was a highly Buddhist document that focus on on the morals and virtues that were to be expected of government officials and the emperor's subjects to ensure a smooth running of the state.    Hence, 593 AD marks (a) the end of the period where Shinto (an the power family backing it) would no longer have a monopoly on religion, (b) the first of many Buddhism rulers, (c) the beginning of more organize rule of the country using Chinese's methods and practices.

Period of Imperial Rule using "the Chinese Model" (593 to1185).  While historically most Japanese have not seen it this way, this period distantly different from the prior period in that the imperial court essentially adopted the "Chinese way" of ruling and administering people.  Hence, even though Buddhism started in India, when you read about "the spread of Buddhism in Japan", you must realize that this phrase is a "code word" for the spread of "Chinese culture" and methods for controlling people.  Remember, that history has proven that it is very difficult to control people unless one has a religion that they can give the people that work in concert with "the government".  Anyway, 600AD was the beginning of an almost 600 year period of imperial rule where the military and civil government is directly control by the powerful families of the imperial court ("and the emperor") using very effective techniques imported from China.  During this time, the imperial court implemented, adopted all the foundational Buddhist doctrine at the beginning of the period (noted above), Chinese as an official court language, the Taika Reform Edicts of 645, and reorganized the government and the penal code in accordance with the Chinese administrative structure - the Ritsuryou defines both a criminal code (Ritsu) and an administrative code (Ryou).  The term Nihon began to be used in his period and uses the Chinese kanji used for this Japanese word means "the origin of the sun" which makes sense only if you are "waking up in China."  The period end with a war over imperial succession - the well know tales of the Genpei War between two powerful families of the imperial court - Minamoto (Genji) and the Taira (Heike). The war against the Taira was lead by Minamoto Yoritomo and resulted in the dramatic destruction and death of many of the powerful imperial court families and the death six-year-old Emperor Antoku at the hands of Minamoto Yoritomo's often hailed brilliant half brother Minamoto Yoshitsune. 

Period of Military Rule (1185 to 1868).  Period where the civil government and administration is controlled by the military leaders and not the families of the imperial court, nor for the matter the emperor. This period has 4 distinct grouping 1) beginning with Minamoto Yoritomo and the "Kamakura (a place) Period", 2) continued with Ashikaga Shoguns - "Muromachi (a place) Period"  3) the short but important rule of Nobunaga - Hideyoshi during the "Azuchi-Momoyama (two places) Period" [also know as the Warring Period or Sengoku Jidai] and  4) ending after the14 successive ruling Tokugawa shoguns - "Edo (a place) Period".  See Unequal Treaties write-up for how this period ended and a good introduction to modern Japan. 

Restoration of Imperial Power, Evolving Democratic Institutions, and Hijacking of the Government by the Military (period from 1868 to 1945)   Officially beginning with the decisive defeat of the Tokugawa Shogun's forces January 1868 and the restoration of rule by the imperial court (Meiji restoration) and ends with the Japanese surrender during WW2. 

Post WWII Government