Faced with a nuclear-armed and provocative North Korea and a newly assertive China, Japan and South Korea rely on US leadership and close military, diplomatic and trade ties with Washington.
If Trump calls this relationship into question -- as he did repeatedly on the campaign stump, before dialing back his rhetoric as Election Day approached -- it will mark a historic geopolitical shift.
Unmoored from Washington, Japan and South Korea may feel obliged to develop nuclear arsenals of their own or make accommodations with US rival Beijing.
If Trump makes good on his protectionist rhetoric or if he labels China a currency manipulator, he could trigger a trade war with Beijing that would destabilize the Pacific economy.
But if he matches his belligerent campaign style with a robust military stance in the region and a diplomatic push to protect allies' interests against China's claims, he could win friends.
North Korea and security
Trump has no foreign policy experience and may have underestimated the alarm his words would cause in Asia when he whipped up partisan crowds in the US heartland when he mocked US allies.
"You know we have a treaty with Japan, where if Japan is attacked, we have to use the full force and might of the United States?" he asked in August at a rally in Des Moines, Iowa.
"If we're attacked, Japan doesn't have to do anything. They can sit home and watch Sony television, OK?" he declared, arguing that the alliance must be a "two-way street."
Also on the campaign trail, Trump suggested that the United States might be "better off" if Japan and South Korea were to build their own nuclear weapons, before furiously insisting he'd never meant to suggest they do so.
Either way, Abe will be looking for reassurance from Trump that Washington will still have Tokyo's back in the event of conflict, especially as North Korea has continued to conduct missile and nuclear weapons tests despite international sanctions.
Trade
President Barack Obama's outgoing administration had hoped to bind its friends on the Pacific Rim -- Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore and Vietnam -- into the Trans-Pacific Partnership trade deal.
With only nine weeks left in his administration, the United States will not now ratify the pact, which appears doomed to fall apart -- a victory for China, which sees itself becoming the region's economic leader.
Japan's lower house did approve the deal, last week, and Abe has made it a pillar of his economic platform, hoping to use preferential access to more developed markets to revive Japan's manufacturing base, which has lost ground to China.
Trump's opposition to the deal was at the heart of his stump speech as he toured blighted former US factory towns in his march to victory, but Abe will be keen to know what can be salvaged from the mess.
China
There is probably no more important bilateral relationship in the world than that between China and the United States -- and Abe will seek reassurances that Trump will help Japan and other Asian allies counterbalance Beijing's pull.
Despite the isolationist drift of his campaign speeches, Trump had harsh words for China, and observers in Washington suggest that if he manages to disentangle US troops from Iraq and Afghanistan, the US military could focus on the Pacific.
Obama's administration has been reluctant to risk confrontation in the South China Sea, where Beijing is building artificial islands to reinforce its disputed claim to a stretch of maritime territory.
Trump may be in a position to extend the US Navy's "freedom of navigation operations" -- operations by warships in waters claimed by China to demonstrate US commitment to keeping sea lanes international.
Japan has its own dispute with China further north in the East China Sea, and Abe will be seeking reassurances that Trump's White House will maintain and strengthen security ties in the Pacific.
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