In his five months as acting secretary, Shanahan has tried to focus on implementing a new national defense strategy that shifts away from fighting extremist groups to what he calls "great power" competition with China and Russia.
But instead, much of his time has been spent juggling a host of other issues: Iranian threats, North Korean missile launches, the ongoing war in Afghanistan, the battle against Islamic State militants, and a divisive struggle to cobble together billions of dollars for Trump's wall on the southern border.
Even as he prepared for the Asia trip, Shanahan shuttled back and forth to the White House for meetings on how much more military might he should send to the Middle East in coming weeks to protect American forces and interests from Iranian threats.
Speaking to reporters traveling with him to Asia on Wednesday, Shanahan acknowledged the competing interests, but insisted he spends "quite a bit of time" on China issues. "Implementation of the national defense strategy is my top priority," he said, adding that the department has "the capacity to spin a lot of plates."
Trump has not yet sent Shanahan's formal nomination as defense chief to Capitol Hill, but he is expected to do so in the coming days. If that happens, the Senate could hold a hearing and vote on Shanahan's confirmation some time later next month.
The acting defense chief's Asia trip will take him to Indonesia, South Korea, Japan and Singapore, where he will attend the Shangri-La Dialogue, a national security conference. And he said it will give him a week "solely dedicated to the issues of the region."
One key meeting at the Singapore conference will be with Chinese Defense Minister Wei Fenghe. For the first time since 2011, China is sending a top level leader to Shangri-La, and it's unclear what triggered the change.
Observers suggest the move underscores Beijing's desire to re-engage with neighbors in the region, perhaps at a time it believes the U.S. is distracted by strains in the Middle East and hampered a bit by Shanahan's status as acting secretary.
Asked about his goals for the meeting, Shanahan said he wants to identify areas where the two nations can cooperate, and "talk about things that I think are important for us to be transparent and candid about."
Tensions between Washington and Beijing have spiked in recent months, as the Trump administration set off a trade war with China, raising tariffs on billions of dollars of imports. The U.S. also sanctioned Chinese tech giant Huawei and approved a weapons sale to Taiwan, the self-ruled island the Communist mainland claims as its own territory.
Two U.S. government reports this year criticized China over its militarization of manmade islands in the South China Sea and its continued campaign to steal high-tech trade secrets from defense programs. And defense intelligence officials expressed worries that China's growing military might could lead to an attack against Taiwan.
Shanahan has repeatedly signaled that he believes America's most pressing security problem is China's rapidly growing military. It is not a new theme. Several of his predecessors pursued what the Obama administration called a "pivot" to the Pacific, all aimed at countering China's growing prominence in the region.
But Shanahan sees it as an increasingly urgent and long-ignored problem, and his proposed budget includes billions of dollars in new programs designed to keep pace with China's strides in hypersonic weapons, nuclear technology and space launches.
A senior U.S. official said there is a lot going on in the world now, and Shanahan's weeklong trip demonstrates that Asia is a priority for the administration. The U.S. will also release a new report in conjunction with the visit, to lay out what the department has done so far to implement the defense strategy.
A former U.S. official familiar with the region said the trip will give Shanahan a chance to show his foreign policy expertise and deliver a speech that continues pressure on China and assures allies of American's commitment to the Indo-Pacific. The officials spoke on condition of anonymity to discuss trip details before they are made public.
Shanahan's chance to deliver his message will come Saturday morning, when he takes center stage at the security conference. How well he does in front of the international audience -- which traditionally includes senior members of Congress -- could also have an impact on his job status.