The departure of Hagel, however, cannot help much with the reshaping of Obama's policy contour for the next two years. Even if Obama became more assertive in the Middle East and East Europe to reconcile his differences with the Congress, it is highly unlikely that he would confront Russia militarily and to send US ground forces back into Iraq to take on the IS. Such actions are not politically realistic or appealing. Instead, they could thwart Obama's fiscal balancing.

So, no matter who succeeds Hagel, the US is likely to remain occupied in the Middle East and Ukraine with little time and resources left to push forward the "pivot to Asia" policy. Obama will obviously defend his next appropriation request, possibly without any more deep cuts in the defense budget, and protect his healthcare program. But on both issues, he is likely to be confronted by the Congress.

The Congress, on its part, would be more concerned about the rise of China and agree in principle to work with the White House to make the "pivot to Asia" policy a success. But for a US on the decline, it would become increasingly difficult to dominate the Asia-Pacific region, the Middle East and Europe at the same time. In fact, Washington could inadvertently push Beijing and Moscow to forge a stronger strategic partnership.

Compared with the burning issues such as the IS and the Ukraine crisis, China's rise at most poses a mid-term challenge to the regional balance of power. The "pivot to Asia" policy of Obama seems poised to hedge against the potential change in the status quo, rather than not allowing China to rise in accordance with international laws.

While Beijing and Washington differ on the applicability of some of these laws, they have already agreed to prevent incidents at high sea, as demonstrated at the meeting between President Xi Jinping and Obama on the sidelines of the recent Asia-Pacific Economic Cooperation conference in Beijing. Still the two countries have a lot to reconcile to avoid incidents within the exclusive maritime economic zone. Therefore, the new secretary of defense is likely to follow Hagel's path to work with Chinese leaders to ally each other's security fears.

The author is a professor and associate dean at the Institute of International Studies, Fudan University, Shanghai.