Final answer:
Mainland China continues to claim Taiwan as part of its territory following the retreat of Nationalist forces there after the Chinese civil war.
Step-by-step explanation:
The country in East Asia that mainland China still claims as an integral part of the Chinese nation is Taiwan. When the Nationalists were defeated in the Chinese civil war, they fled to Taiwan and established an independent government there. However, the establishment of the People's Republic of China in 1949 has led to ongoing diplomatic disputes over the status of Taiwan. China views Taiwan as a renegade province that should be reunified with the mainland, despite the island's self-governance and separate political system. This contest over sovereignty has led to strained relations between Beijing and Taipei, although recent years have seen an increase in trade and economic exchanges.