LOS ANGELES — The two looked over the precipice and gasped at the steepness of the drop. They looked down at a desert of dashed hopes and old skeletons, scraping the bottom of the canyon.

Yes, this is where a failed U.S.-China relationship might wind up.

At first it does look like a very long way down, but gravity would make a fast trip of it. All it would take is a cliff-edge miscalculation or a loss of balance by one or the other for a huge geopolitical slip. China and the United States, the odd couple of the first half of the 21st century, would be at the bottom of the canyon of international stability.