When President-elect Donald J. Trump spoke on the phone with Taiwan’s president on Friday, he was wading into one of Asia’s longest-running and sensitive issues: the dispute between Taiwan and mainland China.

Though the call alarmed experts, who say it risks upending decades of American efforts to manage the dispute, nonexperts could be forgiven for scratching their heads about the uproar. What follows, then, is a guide to the China-Taiwan issue: why it is so delicate, what role the United States has in the matter and why the phone call is significant.

What is the China-Taiwan issue?

Both players claimed, at least formally, to represent all of China — which they considered to include each other’s territory. That created problems, including periodic risks of war, for decades.

The disagreement dates to 1927, when civil war broke out in the Republic of China. The war culminated in Communist revolutionaries, led by Mao Zedong, mostly defeating China’s Nationalist government in 1949.