The CCP is expanding at all costs, and a large part due to population, decreasing resources, and many aspects. The agreement was not as you think with the Ming Dynasty, you may want to rad chuk yeshe kundro and become educated in it. The events of the late 50's was a result of the CCP movement. Before that it was Tibetan hopes of growth in a hard region they lvied in far before Chinese aspects.
I do not believe it is possible for some to understand events in the late 50's, except when you use the emotion and tears of those still alive.
The CCP also does not represent all Chinese people, keep in mind the events were fought on two fronts, but the Communist party gained power.
Many Tibetans died for beliefs and practice on their own lands. This is a factor. They didn't ask for liberation from a political created agenda outside their understanding. One day you were gathering barely bundles,, within 2 months you were turning a majority over to occupying soldiers.
Religion was oppressed. Not only native Tibetans , who some in eastern Tibetan regions married with those of Chinese origin, and shared Buddhist or other beliefs. Chinese practitioners saw beliefs become a historical show and tell for tourism within China itself.
However, in Buddhist views, we have to still have compassion. China was facing economic crises in some aspects. Over population, rising poverty (it still exists in much of china, and some conditions are still very poor). We don't want anyone to suffer, yet nor do we wish our own people. It is a tough situation.
China expanded under the CCP at all costs, claiming liberation, original rights over lands, and not only Tibet. While they didn't, they still made the promises of assisting growth, with intent to wean out existing population, and integrate it into their own. Later it was adjusted when things became more apparent globally. However because of trade, and money, it was over looked.
Don't confuse past unified trade of Tibet and China with rights to Tibet. Tibet has maintained it's individual ownership for a long period of time with some imperial blending, but the Chinese dynasty did not extend that time, the region of the snow was considered unimportant and not viable at that period.
Even in the time of Bon, to the beginning of Buddhism expansion, to Muslim and Mongolian invasion attempts, to the complete teaching and transmission of Vajrayana from india, to the Karmapas, then later Dalai yellow hats, It was maintained. Even with inter-kingdom marriages with china, it was not Chinese control.
Before the late 50's, Tibet was recognized as Tibet, and the CCP who gained control in china, claimed they would liberate the people of Tibet. Prior there was discussion with the Dalai lama to help internal modern growth of Tibet with a seventeen point agreement. China however did not keep with promises, so the need to claim liberation was put into place. Plus don't go but claimed maps, we already know how claims are incorrect. Mao Zedong, chairman Mao, who's ego over came compassion, called "Religion a poison". His lack of Dharma understanding a clear factor.
So compassion and wisdom comes in as factors. While the CCP violated agreements and needed to create a propaganda to convince it's own people of it's intent. China was suffering economic, population, and decreased religious aspirations. Keep in mind, while Buddhism is a large aspect of Tibet, it is not the only religion of Tibet, earlier practice still existed, including bon. Muslims remained in Tibet, and even lived at the foot hills of Lhasa.
The l habitants of Tibet, were attacked for beliefs, respect, and practices. Tibetans told to learn and speak Chinese. Families who owned land, had land taken with poor excuses. Many Tibetans become second rate citizens in their own lands. Railways built to increase incoming flow of Chinese mainland migrants, to begin the process of replacing Tibetans. A forced, systematic control, that is continuing in other areas.
No one from mainland china can understand what it is to Tibetans who became suppressed and second rate, or forced to cross to Nepal, Burma, Bhutan, and other areas to become refugees. To have respected teachers, like karmapas, Rinpoches and lamas escape due to fear of the threats against them, where their only defense is wisdom and compassion. OR schools split because of Chinese interference, and political blend to appease and try to keep peace, comes into play.
Keep in mind how many Tibetan Monasteries were destroyed. Out of thousands, Six remain. How many nuns and monks were killed, imprisoned, threatened. How many Tibetan citizens Threatened, killed, told not to speak Tibetan, to learn Chinese, to be Chinese, to give up Buddhist practices unless told to for appearance.
How is it justified?
The solution was to leave Tibet as it is, discuss sharing of common goals, needs of both people ,and a compassionate solution, not excuses. How many elders, teachers, and wise habitants were ignored? Unless you experienced it you can not understand the impact.
Yet as a result, Buddhism has grown and spread west in a stronger way, and hopefully it will help others grow, and some day, China will benefit once again more from Dharma rather then corrupt communist ideals. Then grow in a positive manner.