China is not the only country trying to enforce questionable claims on maritime real estate. This is actually an old problem and in 2014 the U.S. sent aircraft and warships to challenge 19 such claims by six countries (China, Iran, Philippines, Argentina, Brazil and Venezuela). China gets most of the media coverage in this area but China is not the only one doing it. This has been such a widespread and persistent problem that, once the Cold War ended in 1991, there was finally an opportunity to get this all sorted out and an international treaty was agreed on and signed by the most nations to deal with the endless disputes in this area and all of the resulting harassment of ships and aircraft in international waters. The new treaty (the 1994 Law of the Sea treaty) recognizes the waters 22 kilometers from land as under the jurisdiction of the nation controlling the nearest land. That means ships cannot enter these "territorial waters" without permission. More importantly the waters 360 kilometers from land are considered the Exclusive Economic Zone (EEZ) of the nation controlling the nearest land. The EEZ owner can control who fishes there and extracts natural resources (mostly oil and gas) from the ocean floor. But the EEZ owner cannot prohibit free passage or the laying of pipelines and communications cables.
China is the most frequent abuser of the 1994 treaty, which it signed. For example China claims that American electronic monitoring ships are conducting illegal espionage while in the Chinese EEZ. But the 1994 treaty says nothing about such matters. China is simply doing what China has been doing for centuries, trying to impose its will on neighbors, or anyone venturing into what China considers areas under its control.
China is not alone, but because China is pushing the limits of how the 1994 law can be interpreted (or misinterpreted) other nations with similar opportunities to lay claim to crucial chunks of the seascape are ready to emulate China if some of the more aggressive Chinese ploys actually work.
Standing in the way of all this is the United States, the nation with the largest amount of foreign trade (exports and imports) on the planet and the most aggressive in challenging maritime claims seen by the international community as overly aggressive and basically in violation of the 1994 treaty. For example the U.S. not only points out that the Chinese creation of new islands (by dredging sand from a nearby reef and claiming this as an legitimate extension of a nation’s EEZ) is not legal according to the 1994 law. China has made several other questionable moves and the main obstacle to getting away with it is the American challenges, backed by the support of most of the international community. China has not been dissuaded but has become more deliberate and cautious in making its claims.