Tibet is a mountainous region between India and China proper. At the time when it was invaded by troops of the Chinese People's Liberation Army in 1949, Tibet was de facto an independent state. Within the People's Republic of China, the name is used to refer to the Tibet Autonomous Region, which includes about half of cultural Tibet.
In the 1910s during the fall of the Qing Dynasty, which had more-or-less controlled Tibet, the Tibetans kindly showed the Qing troops in the region the way out. China had its own problems to deal with, with the Warlord Era, the Civil War, and World War II and whatnot, so Tibet was de facto independent for a number of decades.
Before the Communist Party took full power of China in 1949 it began asserting its claim that Tibet was part of Chinese territory and its people were crying out for "liberation" from "imperialist forces" and from the "reactionary feudal regime in Lhasa." It is known that one of the first things that Chairman Mao did when he came to power was send his troops to annex Tibet, which was fiercely anti-Communist and nearly killed him and his little buddies on the Long March, but the Chinese Civil War was actually still in motion after the declaration of PRC's founding, and the campaign for Tibet is a part of it. The People's Liberation Army (PLA) defeated the Tibetan army in a battle at Chamdo on October 7, 1950, marking the beginning of Beijing’s campaign to integrate Tibet into the People's Republic of China. This operation is called a peaceful liberation of Tibet by the PRC government, as the Seventeen Point Agreement was signed by delegates of the 14th Dalai Lama and PRC government affirming Chinese sovereignty over Tibet. It is called an invasion by the Central Tibetan Administration (the Government of Tibet in Exile), the US Congress, military analysts media sources and NGOs such as the International Commission of Jurists and the Center for World Indigenous Studies as the defeated Tibet had little choice but to sign the agreement. Regardless, the Tibetan delegation had not been given authority to sign by their government and so (along with doing so under Chinese duress) it is deemed invalid under international law. The Dalai Lama has repudiated it many times since.
Recently, unrest in Tibet has become more vocal and obvious. There were riots in Lhasa starting since March 14th 2008 and lasted until the 2008, instigated (according to the Chinese) by the Dalai Lama. In reality, it was the 49th anniversary of the invasion, and the Tibetans knew the eye of the world was on China anyway so local dissidents had a few field days slashing and burning people (Chinese, Hui-Chinese Muslims, and dirty, dirty Tibetan collaborators) to death.
“”Tibet seems like as a celestial paradise held in chains, but the west's tendency to romanticise the country's Buddhist culture has distorted our view. Popular belief is that under the Dalai Lama, Tibetans lived contentedly in a spiritual non-violent culture, uncorrupted by lust or greed: but in reality society was far more brutal than that vision.
|—Sorrel Neuss, What we don't hear about Tibet.|
Some people defend the Chinese invasion of Tibet (Battle of Chamdo) by pointing out that the human rights situation has improved. They claim that under occupation, the quality of life for the average Tibetan has greatly improved in comparison to the time of the feudal, theocratic government of the Lamas, when the aforementioned human rights (with the possible exception of self-determination) were also denied, often to a much greater degree. Sorrel Neuss writes in The Guardian's Comment is Free section:
“”Until 1959, when China cracked down on Tibetan rebels and the Dalai Lama fled to northern India, around 98% of the population was enslaved in serfdom. Drepung monastery, on the outskirts of Lhasa, was one of the world's largest landowners with 185 manors, 25,000 serfs, 300 pastures, and 16,000 herdsmen. High-ranking lamas and secular landowners imposed crippling taxes, forced boys into monastic slavery and pilfered most of the country's wealth – torturing disobedient serfs by gouging out their eyes or severing their hamstrings.
|—Sorrel Neuss, What we don't hear about Tibet.|
While this is certainly a valid criticism of the old feudal government, using it to excuse the Chinese abuses could be considered an example of the tu quoque (or in some cases the not as bad as) fallacy. Lhadon Tethong, of Students for a Free Tibet, compares the defense of the occupation with the arguments used in favor of 19th-century European colonial endeavors:
“”The crucial subtext of Beijing's condemnation of Tibet's "feudal" past is a classic colonialist argument that the target's alleged backwardness serves as a justification for invasion and occupation. These are the politics of the colonist, in which the "native" is dehumanized, robbed of agency, and debased in order to make occupation more palatable or even necessary and "civilizing." China has no more right to occupy a "backward" Tibet than Britain had to carry the "white man's burden" in India or Hong Kong.
In its historical context, the invasion into Tibet is an extension of the Chinese Civil War – a class war/mob action which does not base its intentions on culture, ethnicity or religion, but wealth. It is still questionable whether the Communist People's Liberation Army is entitled to mount an offensive into a territory its enemy does not hold de facto, but they believed they were liberating fellow workers and peasants regardless of any borders.
Human rights abuses documented in Tibet include the deprivation of life, disappearances, torture, poor prison conditions, arbitrary arrest and detention, denial of fair public trial, denial of freedom of speech and of press and Internet freedoms. They also include political and religious repression, forced abortions, sterilisation, and even infanticide. Since being recognized at a young age, Gedhun Choekyi Nyima recognized by the Dalai Lama as the 11th Panchen Lama has along with his family been kept in isolation since 1995 under house arrest. Attempts outside the Chinese government to confirm his health and well being have been attempted by Mary Robinson, the U.N. High Commissioner for Human Rights; Harold Koh, the U.S. Assistant Secretary of State for Democracy, Human Rights and Labor; and Raymond Chan, the Canadian Secretary of State for Asia and the Pacific, but each have been denied access. Human Rights Watch estimates that there are approximately 600 known political prisoners in Tibet, mostly clergy; Tibetan students studying abroad have also been detained upon return. 
In addition, there is also a degree of animosity between ethnic Tibetans and Hui Muslims. This dates back to the 1930s, when Hui Muslim warlord Ma Bufang waged a series of wars against the 13th Dalai Lama, in the former's attempts to create a Muslim enclave in nearby Qinghai province, which forced many Tibetans off their lands. Tensions were suppressed by the CCP government after the 1950 invasion, but resurfaced in the 1990s, after the economic liberalization of China relaxed many travel restrictions, which led to many Muslims migrating into Tibetan areas. Sectarian violence had flared up in the following decades, particularly during the 2008 Tibet riots, and as a result, many Hui Muslims tend to support the Chinese government's repression of Tibetan separatism.
It should be noted that the Chinese government does not support the Universal Declaration of Human Rights or guarantee these rights to any of its citizens. While living conditions in Tibet may be worse than in most of the country, citizens throughout PRC do not enjoy full freedom of speech, assembly or movement. Beijing's line is that human rights are China's internal affair.
However, it is important for Westerners to keep in mind that pre-1959 Tibet was emphatically not the peaceful, egalitarian Shangri-La nation of happy monks that Richard Gere and other white Buddhists try to portray. The Dalai Lamas of old did in fact rule Tibet with an iron fist, and most people, if not serfs or slaves outright, were nonetheless oppressed under a caste system that privileged wealthy elites and monks over ordinary peasants (and this is not counting the many people, especially in U-Tsang, who were agricultural slaves). Fighting Chinese propaganda with hippy-dippy western Buddhist propaganda will not do anything to solve problems in this region of the world, and only serves to infantilize the Tibetan people into some idealistic pipe dream of bored, bourgeois white people of how they think a quaint, happy mountain people ought to behave.
- "Peaceful Liberation of Tibet"
- The Status of Tibet
- US Congressional Concurrent Resolution dated May 21, 1991, "To express the sense of the Congress that Tibet, including those areas incorporated into the Chinese provinces of Sichuan, Yunnan, Gansu and Qinghai that have historically been a part of Tibet is an occupied country under established principles of international law" ... "Whereas in 1949-50, China launched an armed invasion of Tibet in contravention of international law..."
- Reborn supremacy: China's control of Tibetan reincarnation, Jane's Intelligence Review
- International Commission of Jurists, June 5, 1959, "In 1950 China assured India that China had no intention of incorporating Tibet into China by force or otherwise and was willing to negotiate with Tibet regarding the future relationship of Tibet with China. But a few weeks later the invasion of Tibet took place..."
- Missions to Tibet, University of Texas, Dallas, May 29, 2006: "When Communist China invaded Tibet in August 1950...."
- Global Security: "On October 1, 1949, the People's Republic of China was formally proclaimed in Beijing and the following year launched an armed invasion of Tibet..."
- Tibet's Sovereignty and the Tibetan People's Right to Self-Determination, June 1, 1998
- Richter, Conrad. "Tibetan Response to China's White on Tibet." Center for World Indigenous Studies, Olympia, WA, 1999
- Neuss, Sorrel. "What we don't hear about Tibet", The Guardian (Comment is Free). (Wednesday 11 February 2009 22.00 GMT)
- Students for a Free Tibet: China’s Favorite Propaganda on Tibet…and Why It’s Wrong by Lhadon Tethong
- "The Historical Nature of China's Tibet: (3) The Local Government of Tibet Refused Peace Talks and the PLA Was Forced to Fight the Qamdo Battle" China Intercontinental Press. Hosted at china.com.cn
- US State Department, Bureau of Democracy, Human Rights, and Labor, 2009 Human Rights Report: China (includes Tibet, Hong Kong, and Macau), March 11, 2010
- Regions and territories: Tibet BBC News
- NGO Group for the Convention on the Rights of the Child Database of NGO Reports presented to the UN Committee on the Rights of the Child - Genocide in Tibet - Children of Despair - Introduction by Paul Ingram "there is a great deal of evidence and detailed testimony, which indicates that this [forced abortions, sterilisation] has been Chinese policy in Tibet for many years"
- Goldstein, Melvyn; Cynthia, Beall (March 1991). "China's Birth Control Policy in the Tibet Autonomous Region". Asian Survey 31 (3): 285–303. JSTOR 2645246.
- "Human Rights Violations in Tibet", Eliot Sperling, Human Rights Watch, June 13, 2000
- Tibetan-Muslim tensions roil China LA Times
- Tibet our internal affair, US told
- Beijing warns U.S. lawmakers to stay out of China's internal affairs
- Just how far can 'internal affairs' be stretched?: "Ever since the 1950s, China has subscribed to the principle of non-interference in the internal affairs of other countries, which was first written into a treaty that it signed with India in 1954. Since then, China has continued to loudly uphold this principle and to criticise those who, in Beijing's view, interfere in its internal affairs, including commenting on its human rights record."