Prior to emperor status, Emperor Xian was required, and you could only annex Han colonies one at a time. When you acquire the status of emperor, your vassalage with Han and his master is terminated. After that, you must vassalize Han once more before annexing him all at once. This process can be done multiple times.
As for how to annex the Han dynasty: you need to build up your own empire first, then annex them. However, it is possible to speed up this process by using an advisor who will tell you to destroy their military or surrender terms will be issued to you. You have 3 options here: either destroy their army, become their vassal, or accept their terms (which results in no action being taken). If you choose not to follow any advice, you can still end up annexing them anyway after a few years if you aren't doing anything else with your time.
What was it like for the Han emperors to oversee their huge empire? To avoid having to share authority, they made all decisions themselves. The government was managed by a massive, sophisticated bureaucracy. They collaborated with local administrations led by their male kin. And they relied on military power to maintain order throughout the realm.
The Han dynasty was one of the most successful in Chinese history. Their territory increased more than any other dynasty's before or after them, and they ruled over a population of hundreds of millions. But they failed to produce a single world-class ruler. All the Han emperors were talented politicians who assembled teams of advisors who helped them make decisions. But none of them had enough strength or willpower to dominate their courtiers instead of being dominated by them.
The empire was divided into four parts: China proper, which included the central plains but not modern-day Mongolia; Northern Vietnam; Eastern Europe; and South Asia. Within each region, there were several large provinces that were further subdivided into counties and commanderies. There were also smaller territories such as Petra (in present-day Jordan) that belonged to the emperor but received legal protection from larger states. These separate pieces of land were tied together only by treaty or military force. No single ruler could control them all.
In addition to administering their empire, the Han emperors had to fight numerous wars against rival nations and internal rebellions.
What impact did a string of weak rulers have on the Han dynasty? - Foreign countries were able to seize control of vast swaths of territory. Rivalries among prominent landowners splintered the empire. The emperors' several heirs generated rivalry for the throne. And many ordinary people looked to escape punishment by sacrificing others instead of themselves.
The story of the Han dynasty is the story of one long struggle for power between two opposing forces: those who support the monarchy and those who support democracy. During its nearly 200-year existence, the dynasty survived numerous challenges from within and without its realm. These include civil wars, foreign invasions, economic chaos and depression.
The dynasty's fate was decided in the palace courts where powerful officials sought to advance their own careers by bringing down their rivals. The struggle was often brutal; prisoners of war were usually executed after a short trial. Even if they were given a royal pardon, victims were often exiled from the capital or forced to work as slaves on large estates owned by high-ranking officials.
During the reign of Emperor Wu (ad 1-AD 24), the dynasty's last years were marked by turmoil and unrest. He tried to resolve conflicts among the nobility by creating new offices that he could hold himself but this only added to the chaos. In AD 7, he abandoned his attempt at ruling personally and left the country for exile in southern China.
The Han dynasty quickly returned feudal lords to positions of authority. The early Han dynasty's greatest achievements revolve around Wu Ti, the first emperor to govern under the Mandate of Heaven. According to the commandment, emperors were subject to the sovereignty of heaven. Their success was predicated on the gods' approval. If the gods found that they had fallen away from their duties and made any missteps, they would take revenge by removing their blessings--the emperors' prosperity and security. This belief system was intended to prevent overreaching by the emperor and ensure his or her accountability before God.
China's traditional ideology is based on five key concepts: heaven, earth, man, government, and culture. Heaven refers to the divine realm above, while earth represents the natural world below. Man is seen as having two parts: body and soul. The body belongs to earth, while the soul resides in heaven. Humans are thus a part of both worlds and need to fulfill their obligations in each; failing to do so will result in punishment in the next life.
Feudalism was introduced to China during the Han dynasty. Under this system, local rulers were given authority by the central government to administer their own regions. They were responsible for maintaining public order and collecting taxes within their territories. If these duties were not done properly, they could be punished by higher-ranking officials up to and including the emperor himself.