It was first used to refer to the dominant position of the Ottoman Empire in relation to its surrounding regions; the Ottoman Empire being the suzerain, and the relationship being suzerainty. The terminology gradually became generalised to refer to any relationship in which one region or people controls the foreign policy and international relations of a tributary state, while allowing the tributary nation to have internal autonomy. Modern writers also sometimes use the term suzerain to refer to a feudal lord, in regard to their relationship to their vassals.
Suzerainty differs from true sovereignty, as the tributary state or person is technically independent, and enjoys self-rule (though usually limited in practice). Although the situation has existed in a number of historical empires, it is considered difficult to reconcile with 20th- or 21st-century concepts of international law, in which sovereignty either exists or does not. While a sovereign nation can agree by treaty to become a protectorate of a stronger power, modern international law does not recognise any way of making this relationship compulsory on the weaker power. Suzerainty, therefore, describes a practical, de facto situation, rather than a legal, de jure one.
Historically, the Emperor of China saw himself as the centre of the entire civilised world, and diplomatic relations in East Asia were based on the theory that all rulers of the world derived their authority from the Emperor. The degree to which this authority existed in fact changed from dynasty to dynasty. However, even during periods when political power was distributed evenly across several political entities, Chinese political theory recognised only one emperor, and asserted that his authority was paramount throughout the world. Diplomatic relations with the Chinese emperor were made on the theory of tributary states, although in practice tributary relations would often result in a form of trade, under the theory that the emperor in his kindness would reward the tributary state with gifts of equal or greater value.
This system broke down in the 18th and 19th centuries in two ways. First, during the 17th century, China was ruled by the ethnically Manchu Qing Dynasty which ruled a multi-ethnic empire and justified their rule through different theories of rulership. While not contradicting traditional Han Chinese theories of the emperor as universal ruler, the Qing did begin to make a distinction between areas of the world which they ruled and areas which they did not. Second, the system further broke down as China was confronted by European powers whose theories of sovereignty were based on international law and relations between separate states.
A series of "unequal treaties" (including among others the Treaty of Nanjing, 1842; the treaties of Tianjin, 1858; and the Beijing Conventions, 1860) forced China to open new ports, including Canton, Amoy, and Shanghai. They allowed the British to set up their own colony at Hong Kong and established international settlements in these ports that were controlled by foreigners. They required China to permanently accept diplomats at Peking; provided for free movement for foreign ships in Chinese rivers; imposed European regulation of Chinese tariffs; and opened the interior to Christian missionaries. Ever since the 1920s the "unequal treaties" have been a centerpiece of Chinese grievances against the West.