China

The People's Republic of China, also simply known as China, is the second largest nation on Earth in the terms of population (behind that of the Republic of India) and one of the largest members of the Coalition of Congressional Republics. Alongside nations such as the United States of America, China is one of the core founders of the CCR, and is one of the few Terran nations to have territorial possessions beyond that of the Solar System. With a population of 30 billion people and a history spanning for millennia, China has enshrined itself as one of the most influential nations in all of human history—and continues to be so.
The modern form of China started with the Proclamation of the People's Republic of China in October 1st of 1949 by Mao Zedong, bringing an end to the Chinese Communist Revolution and the rise of the new state. China would then become an integral ally of the Soviet Union up until the Sino-Soviet split of the 1960s, where it would eventually lead to the formation of a tri-polar Cold War and the consolidation of Chinese hegemony on the Asian continent. However, by 2008 (and with the expedient growth of technology and space development by human powers), China joined the Soviets, Americans, and Europeans in the signage of the Vienna Accords in order to usher in a new, peaceful age of progress.
Even so, problems began to arise within this "period of peace", as in 2049 the First Martian-Terran War began. Chinese possessions on Mars fell to independence movements within days, and as such the PRC joined the US, EU, and USSR in launching punitive offensives against the upstart Martian colonies. Even so, the combined UN forces would eventually lose in their war against the Martians, resulting in the Treaty of Luna in 2052 and the perception of the UN as a now antiquated force. 
China, having resolved its historical rivalry with the Soviet Union, joined forces to create the new Harbin Pact in 2054 as a means to counter the return of the North Atlantic Treaty Organization (NATO) and the Southeastern Asia Treaty Organization (SEATO), both being commanded by the triaxial alliances between Japan, the United States, and the European Federation. With NATO and SEATO merging to form the Global Democratic Treaty Organization (GTDO) in 2055, geopolitical relations across Earth and the wider Solar System gradually deteriorated.
The Third World War began on October 1st of 2065 as a result of China's invasion of the island of Taiwan, which served as the lynchpin of the GTDO's Asian defenses. On October 2nd, as the members of the GDTO officially declared war on China, the European Federation staged a coup d'etat in the condominium of the Republic of Germany as a means to gain further initiative against the inevitable entry of the Soviet Union, thus beginning the global war in full.
WWIII would end with millions dead on both sides in late 2068 as a result of war weariness and the mutual fears of the increasing power of the Alliance of Independent States of Mars; China, having suffered major infrastructural damage, gradually rebuilt. During wartime, the Chen Practical Industries corporation (a predecessor to the modern Chen Wu Corporation) was awarded a multitude of military contracts, thus marrying it to the auspices of the Chinese government and later cementing its future ascent as humanity's largest corporation.
China would prove instrumental to the United Nations' war effort in 2074 with the outbreak of the Second Martian-Terran War, mobilizing its massive industrial capacities and military to fight against the qualitatively superior forces of Mars (and later that of the Sol Outer Colonies Association). With the war's end in 2077 and the signing of the Solar Accords between all involved nations, China was recognized as one of the core founders of the new Coalition of Congressional Republics. Following the inception of the faster-than-light Tepiltzin-Csanád Drive in 2083, China and its native megacorporations would prove key to the rise of humanity's new interstellar empire, with the PRC near its center.
China currently possesses the most densely-populated area in all of the Coalition, the Pearl River Delta Megasprawl, which interfaces with the most densely-populated nation in the CCR—that being the semi-autonomous Republic of Hong Kong and Macau. The RHKM was issued a high degree of autonomy from China in all but economy and military matters in 2077 as a means to help build the "capital" of the Coalition, which exists in Hong Kong as the Central Headquarters of the Coalition, and to cultivate a positive reputation with the Coalition government proper.