It has been 48 years since Henry Kissinger, then national security adviser to U.S. president Richard Nixon, made his top-secret pilgrimage to Beijing to prepare for an historic meeting between Nixon and Chinese Communist Party chairman Mao Zedong.

The two leaders were sworn Cold War enemies. But Nixon’s visit to China in February, 1972 reversed a quarter century-long standoff between their nations. That meeting, which Nixon later called “the week that changed the world,” laid the foundations for the modern global economy’s most significant—and contentious—partnership.

So it was extraordinary to see Kissinger, now 96, back in Beijing this week, and reflecting on the evolution of the U.S.-China relationship at the Bloomberg Next Economy Forum. In a conversation with historian Niall Ferguson, Kissinger acknowledged that, when he recommended Nixon’s historic overture, he had known relatively little about China other than that it was big, and that the logic of realist political theory suggested the U.S. should be “closer to China than China was to Russia.”

Five decades on, Kissinger sees China as America’s only strategic rival—and potentially a far more dangerous nemesis than the Soviet Union. He told Ferguson the U.S. and China are in the “foothills of a Cold War,” and the conflict between them, if left unresolved, could prove far worse than the first World War.

Kissinger’s comments came as U.S and Chinese negotiators remain deadlocked on trade issues. On Friday Chinese president Xi Jinping said Beijing hopes to thrash out a ‘phase one’ trade pact with the U.S.—but is not afraid to retaliate if necessary.

Hours later, President Trump said a trade deal with China is “potentially very close”—but insisted any agreement would have to be weighted in favor of the U.S. to rectify years of trade imbalances with China.

Kissinger said he hoped trade negotiations would succeed. But he cautioned that a trade deal “can only be a small beginning” to a broader political discussion between the superpowers.