Sunnylands, or the former Annenberg Estate in Rancho Mirage, California, was little known to the world until Chinese President Xi Jinping and US President Barack Obama met there for an informal summit early last June.

The two leaders, one who had just assumed the presidency in China and the other who had just started his second term, agreed to meet because they could not wait for a possible meeting on the sidelines of the G20 summit in Russia in September 2013.

Although they had already met during Xi's trip to the US as China's vice-president in February 2012, it was very encouraging for China watchers to see the two leaders trying to understand each other better in a shirtsleeves summit. The symbolism of this was profound as rather than presenting it as the traditional rivalry between existing and emerging powers, it set the tone for a new type of major-country relationship that aims to expand cooperation and effectively manage differences.

I have heard on numerous occasions from senior government officials of both countries that they are working hard to implement or reflect the Sunnylands spirit.

One of the most tangible signs that progress is being made is the fast growing bilateral military exchanges, which for a long time lagged behind the cooperation in other areas.

China's top military officers, from Defense Minister Chang Wanquan, Chief of the People's Liberation Army General Staff Fang Fenghui and PLA Navy Commander-in-Chief Wu Shengli, have visited the US and toured military facilities in the past year. US Defense Secretary Chuck Hagel and several top military leaders from the US Army, Navy and Air Force have also visited China and toured PLA facilities.

The two militaries have conducted joint disaster relief and humanitarian assistance exercises. And China's decision to accept the invitation to participate in the Rim of the Pacific naval exercise for the first time is regarded a major step in building military-to-military trust.

Such progress after Sunnylands has been heartening. Yet what has happened in the last few months has been disheartening and has not reflected the Sunnylands spirit.

There is no doubt that differences, huge differences, exist between China and the US. While the potential for cooperation is limitless, their competition in the economic and security arenas is growing. Especially as the two countries are still suspicious of each other's strategic intentions, whether in terms of the US' rebalancing to Asia strategy or China's growing economic and military might.

The ongoing tension in the South and East China seas which the US is trying to get itself involved in has also complicated the situation, so has the row over cybersecurity, with the two countries trading accusations and adopting a tit-for-tat approach following the US Justice Department's indictment of five PLA officers for cyber theft.

While no one should underestimate the differences between China and the US, what has been lacking is a second and third Sunnylands summit as many suggested a year ago. The most consequential bilateral relationship in the 21st century deserves more attention from the two leaders so they engage in substantive dialogue and more summits like the one at Sunnylands.

The top leaders need to repeat the tone they set at the Sunnylands to reassure everyone that China and the US are heading in the same direction, toward cooperation and not confrontation. This is especially important now that the relationship has hit a snag.

The two top leaders must continue to show their visionary leadership, just as they did at Sunnylands last June.

The author, based in Washington, is deputy editor of China Daily USA. [email protected]