With Sunday marking the second anniversary of Donald Trump's presidency, the United States' alliance with Japan has generally strengthened but his "America First" agenda -- ranging from trade to North Korea -- has sometimes generated uncertainty in otherwise firm bilateral relations.

Personal rapport between Trump and Japanese Prime Minister Shinzo Abe as well as shared concerns about China's rising assertiveness in the Indo-Pacific region have bolstered Washington-Tokyo ties, according to U.S.-based Japan experts.

"The Donald-Shinzo bond seems genuine, and Prime Minister Abe has tapped on this personal connection to prevent the sharp deterioration of bilateral relations that many feared was inevitable given President Trump's obsession with bilateral trade deficits and deep skepticism of the value of alliances," said Brookings Institution scholar Mireya Solis.

A broad alignment between the two leaders on Asia policy was evident in Trump's embrace of the vision of a free and open Indo-Pacific, a Tokyo-led concept widely seen as a counter Beijing's aggressive posture such as militarization of the South China Sea and what critics call "debt-trap" diplomacy toward developing countries.

"Given Japan's deep concerns with China's increasing assertiveness in the neighborhood and its military buildup, the more muscular approach promised by the Trump administration through its 'peace through strength' motto sent reassurances to Tokyo," said Solis, director of the Center for East Asia Policy Studies at the Washington think tank.

But Trump initiatives such as the U.S. withdrawal from the Trans-Pacific Partnership trade deal, the suspension of U.S. military exercises with South Korea and the imposition of tariffs on steel and aluminum imports from Japan and other U.S. allies have dealt blows to the Abe government.

In an apparent criticism of the America First mantra, only 28 percent of Japanese respondents to a 2018 U.S. survey said Washington takes Tokyo's interests into account when making international policy decisions, down from 38 percent in 2013 under former President Barack Obama.

In light of the U.S. pullout from the TPP and exit from the Paris climate agreement, the Pew Research Center survey found that only 30 percent of Japanese have confidence in Trump's handling of world affairs, compared with a range of 60 to 85 percent under the Obama presidency.

Citing concerns about planned negotiations for a Japan-U.S. trade agreement and Trump's dealings with North Korean leader Kim Jong Un, experts believe uncertainty in Tokyo-Washington relations is likely to increase as the Republican president steps up his America First agenda in his bid for re-election in 2020.

With Trump and Kim set to hold a second summit "near the end of February," some experts fear the leaders may agree that Washington would ease sanctions in exchange for Pyongyang addressing the issue of intercontinental ballistic missiles that could strike the United States. Such a deal, however, would leave the issue of shorter range missiles capable of hitting Japan and South Korea unresolved.

The experts also worry that given Trump's decision in December to pull U.S. troops out of Syria -- a surprise that prompted the resignation of Jim Mattis as defense secretary -- Trump might announce a possible withdrawal of American troops from South Korea, a development that would significantly affect the security of Japan.

"The theatrical back and forth between Trump and Kim Jong Un has left Japan feeling sidelined and nervous about a potential deal that might be beneficial for the U.S., but not for Japan," said Sayuri Romei, fellow for security and foreign affairs at Sasakawa Peace Foundation USA, a Washington think tank.

After the first-ever U.S.-North Korea summit last June in Singapore, Trump said, "I want to bring our soldiers back home," in reference to 28,500 American troops stationed in South Korea.

Together with Trump's suspension of joint military exercises with Seoul -- a major concession to Kim in the absence of Pyongyang's credible measures for denuclearization -- Romei said, "Unpredictability and surprise are the things that allies fear the most, and I think Japan has felt that in the past two years."

"The feeling of being left out is not necessarily expressed publicly by Japanese officials, but I have heard some frustration in more private settings," she said.

Uncertainty runs deep in the economic field, as well. Japan is insisting the two countries aim for a trade agreement on goods only but the United States is pushing for a comprehensive pact that would also cover areas such as services, investment, intellectual property and currency as a means to reduce the trade deficit with Japan.

Referring to upcoming trade talks, the Trump administration has said it "may seek to pursue negotiations with Japan in stages, as appropriate" -- indicating the possibility of first reaching a smaller deal that would not cover a full range of issues.

"Clearly, the U.S. wants an early agreement on agriculture to appease farmers and will likely push Japan for an export restraint on automobiles," said Brookings' Solis.

Trump regards automobiles as a symbol of the trade imbalance with Japan, apparently because automobiles and auto parts accounted for about 75 percent of the U.S. deficit with the country as of 2017.

Solis believes Japan will reject export quotas because it does not want managed trade. "So how quickly the two sides can reach a deal on market access is uncertain," she said.

Solis added it is also difficult to see how possible phased talks will actually unfold, given Tokyo's insistence that it has no plans to include a currency provision in a trade deal, partly because such action could affect monetary policy by the Bank of Japan.