US-China relationship complex: former top US official The ability of China and the US to co-exist in the Asia Pacific region was the most important foreign policy challenge of the present century, former US assistant Secretary of State Kurt Campbell declared on Thursday. "It is extraordinarily important that we find ways to work together" he told an audience of foreign policy specialists in Sydney. The US-China relationship was the "most complex relationship that we have ever had and that we will ever have." Finding "contours of coexistence" would be very difficult but "both leaders deep down on each side .....understand that it is necessary" he said. It was "inconceivable that we could find ourselves in a set of circumstances of sustained endemic tension between the United States and China." Mr Campbell, until last month one of the key foreign policy advisers to Hillary Clinton, was speaking as part of a Sydney University US Studies Centre program on the US -Australian alliance. It is his first public address since recently leaving his post as US Assistant Secretary of State East Asian and Pacific Affairs. He said the current territorial dispute between Beijing and Tokyo over the disputed Senkaku Islands called for "cool heads" to prevail on all sides. The US role required it to be " very very careful" he said. Mr Campbell would not be drawn on reputed concern in Washington on the Gillard government 's deep Defence cuts but said there had been "quiet discussions" with Australian officials. "The lion's share of the history of the twenty first century will be written in the Asia Pacific region " he said. We must do everything we can to pivot,rebalance, choose your appropriate word ..to focus on this incredibly important region. Mr Campbell praised former prime minister Kevin Rudd for encouraging the US to join the East Asia summit. "What we have found perhaps more than any other country is the ability of Australia at key junctures to give remarkably helpful and insightful advice," he said.