BEIJING — The Chinese leader, wearing a dark Mao suit, and the American president, in a black tuxedo, stood side by side with arms aloft at the Kennedy Center. Deng Xiaoping and Jimmy Carter smiled broadly as the orchestra played “Getting to Know You,” signaling the dawn of a new era of friendship and cooperation between their two nations.

Over the next 40 years, China and the United States built the most important economic relationship in the world and worked together on issues such as regional security, counterterrorism and climate change. Taking Mr. Deng’s lead, China played the junior partner, if not always deferential then at least soft-pedaling its ambitions and avoiding conflict with the much stronger United States.

Now, faster than many in either nation expected, that has all changed.

On Monday, the United States will begin taxing $200 billion in imports from China, the biggest round of tariffs to take effect yet in an escalating trade war. President Trump says the measures are necessary to fight an economic model that requires American companies to hand over technology in exchange for market access and provides state subsidies to Chinese competitors.

China’s strongman leader, Xi Jinping, presiding over an economy gaining quickly on the United States, has openly challenged American leadership abroad while dashing hopes of any political thaw at home. During this time, both Republicans and Democrats in Washington have turned on Beijing, accusing it of imperial ambitions in Asia, aggression in disputed waters, persecution of ethnic minorities and unscrupulous trade policies aimed at dominating the industries of the future.