President Donald Trump may have alienated allies across Asia but he has, at least, arrested a downward spiral in bilateral relations between the U.S. and the Philippines.

By openly embracing strongmen like Rodrigo Duterte, he has given the Philippines president a prop at a time when other traditional partners, notably in Europe, are holding their noses at his human rights record.

However, despite the Trump-Duterte bromance, what was once considered, in Manila, as a special and sacred partnership has now been totally transformed into an arms-length relationship. It is a largely transactional arrangement that can be strained at any time by the two mercurial leaders in Washington and Manila.

Both Manila and Washington need each other to check China's maritime ambitions as Beijing pushes its claims -- and its ships -- into waters disputed by the Philippines.

The Philippines provides unique geographical location at the intersection of the Western Pacific and the South China Sea, while the U.S. is the only power capable of matching China's military muscle. Unless the two allies work together, Beijing's domination of one of the world's most important waterways will become inevitable.

Duterte's 2016 election victory came largely as a shock to the U.S. Before the vote, many American officials were largely dismissive of his fiery rhetoric, viewing Duterte's promise of a more nationalist foreign policy as sensationalist campaign antics.

They remained confident that Philippine-U.S. bilateral relations were almost immune to shifting sands of politics. After all, the Philippine political elite has been historically subservient to American regional interests.

Throughout the Cold War, major Philippine bases such as Subic and Clark (which American troops have long since left) served as forward deployment positions for the U.S. In more recent years, Manila was among the most enthusiastic supporters of Barack Obama administration's pivot to Asia policy.

Duterte's disavowal of the U.S. and switch to closer ties with Beijing astonished Washington.

"I will be chartering [sic] a [new] course [for the Philippines] on its own and will not be dependent on the United States," he declared days after his landslide victory.

The tough-talking Filipino leader openly called for alliances with the U.S.'s chief rivals, Russia as well as China, while lashing out at the West for criticizing his human rights record.

Two factors drove Duterte's disruptive foreign policy bent. One was his ideological antipathy toward Washington, rooted in his socialization as a student at the height of the Vietnam War.

Later, as mayor of Davao city, on Mindanao, Duterte experienced an often-fraught relationship with the U.S., opposing the American military presence in Mindanao as a violation of Philippine sovereignty.

Yet, Duterte also views the U.S. as a declining superpower, incapable of helping the Philippines address its primary developmental needs.

The Obama administration's persistent ambivalence on its willingness to come to the Philippines' rescue in an event of conflict with China in the South China Sea seemed to confirm Duterte's concerns. "Are you with us?" he once declared, openly questioning Washington's commitment to Philippine security.

His foreign policy recalibration was undoubtedly made easier by Obama's failure to intervene early on and prevent China's move to militarize the Spratly Islands in late-2013.

In contrast, Duterte saw China as a geographical reality and a rising power, which has resources that are indispensable to the Philippines' national development.

But Trump's election in late 2016 boosted bilateral ties. Obama's successor vowed not to impose American values on foreign nations, downgraded the emphasis on human rights issues, and, at times, openly praised Duterte's violent drug war as an "unbelievable job."

Trump and Duterte held cordial meetings on the sidelines of the Association of Southeast Asian Nations in late 2017, vowing to maintain strong cooperation in counternarcotics and counterterrorism operations.

Trump celebrated their "great relationship" and described the meeting as "very successful." According to Filipino officials, the American president did not raise human rights issues during the bilateral summit, much to Duterte's delight.

Amid the siege of Marawi by Islamic State-affiliated groups, Duterte welcomed American assistance including state-of-the-art weapons, precious intelligence, and urban warfare training.

But on the crucial issue of Chinese maritime expansion, there is a big gap. Duterte has consistently downplayed China's reclamation and militarization activities, told external powers like the U.S. that the disputes are "better left untouched," and is exploring closer military ties with China, including proposals for joint naval war games in contested waters.

While some Manila foreign policy officials have serious reservations, Duterte has often expressed his "love" for China, described the Asian powerhouse as his personal "protector," and extols the virtue of being "meek" and "humble" in exchange for Beijing's mercy.

Crucially, he has rejected a U.S. request to develop and preposition weapons in strategic installations such as the Bautista Airbase in Palawan, close to the Spratly chain of islands.

Washington has not managed to fully optimize the Enhanced Defense Cooperation Agreement (EDCA) with the Philippines, which was signed under the previous administrations with the clear purpose of checking China's maritime ambitions.

This hampers Washington's ability to respond to possible contingencies in the South China Sea, including aiding the Philippines in an event of conflict with China. It also undermines the United States' ability to maintain robust naval footprint in the area as a latent deterrence against China's growing maritime ambitions.

To be fair, Duterte's pro-China swing was facilitated by Obama's reluctance to take on Beijing. The Trump administration has adopted a more robust response to Chinese expansionism through enlarged naval operations, in which ships sail through disputed waters to assert freedom of navigation.

Yet Washington is increasingly distracted by an intensifying trade war with Beijing and with key allies as well as a huge diplomatic gamble over North Korea, which both require strategic bandwidth. Nor has Trump's mercurial character and unpredictability inspired much confidence.

The Philippines and America have struggled to effectively rein in Beijing's rapidly expanding footprint across the contested waters. In the scramble at the heart of maritime Asia, the two countries must hang together or risk being hung apart. If they do not fully operationalize their alliance they will strengthen China's hand, and not only in the South China Sea.

Richard Heydarian is a Manila-based academic, columnist and author; his latest book is "The Rise of Duterte: A Populist Revolt Against Elite Democracy."