Philippine President Rodrigo Duterte latest visit to Japan, his third in less than three years, highlights the recent deepening of a long-standing strategic partnership.

Bilateral relations have never been as good since the end of Second World War, amid a long-term convergence of strategic and economic interests between the two Asian countries.

This blossoming relationship is driven not only by old-established mutual economic interests, but also the more recent upsurge of shared concerns over China's maritime expansionism in the East China and the South China seas.

Despite international criticism of Duterte over human rights, the two nations are democracies. Moreover, both are American treaty allies now facing the challenges of an increasingly unpredictable White House, which is committed to opposing Chinese expansion at the same time as showing some signs of reducing its economic engagement in Asia.

The ties are personal as well as political. Duterte is a former provincial mayor who enjoyed decades-long cordial relations with the Japanese consulate and Japanese investors in his city of Davao. He considers Tokyo his most reliable friend.

He is pals with Japanese Prime Minister Shinzo Abe, who was the first foreign leader to visit the Philippines in January 2017 after Duterte's election and even traveled to Duterte's home in Davao with his wife.

The Filipino president said at the time, "Japan is a friend closer than a brother. That means Japan is a friend unlike any other." The Japanese leader then played a crucial role as an interlocutor in diplomatic spats between Manila and Washington over human rights concerns triggered by Duterte's bloody war on drugs.

As one senior Japanese official told me at the time, Abe expressly advised both U.S. President Donald Trump and Duterte to focus on shared concerns rather than disagreements over human rights.

Warm bilateral relations are popular with the Philippines public, with surveys consistently putting Japan as the second most trusted nation just behind the United States. The Japanese public are not often asked about their views of the Philippines but surveys do show that Japanese people share Filipinos' strategic concerns about Chinese expansionism.

Ostensibly, Duterte visited Japan as the keynote speaker for Nikkei's Future of Asia conference, which brought together officials, business people and journalists from across the region.

But the timing and scale of Duterte's large delegation was eye-catching. The visit came just weeks after the Filipino leader's latest trip to China. Back in October 2016, Duterte also made a similar move, visiting Japan just days after his state visit to Beijing. This way, he made it clear that his priority is closer ties with neighbors -- and not China per se.

The size of the 200-strong entourage invited controversy at home, but highlighted the importance Manila attaches to its ties with Tokyo.

The Filipino president is clearly right to diversify his strategic relations, bearing in mind the challenges now posed by both Beijing and Washington.

By pivoting to Japan, the Filipino leader has sought to mitigate the Philippines' economic dependence on China, while finding alternative security partners beyond traditional Western allies, which have been openly critical of his human rights record.

China's maritime assertiveness is perhaps his top concern. Speaking at the Nikkei forum, the Filipino president questioned whether "is it right for a country [China] to claim the whole ocean [the South China Sea and Western Pacific]?"

Over the past few years, Japan has sought to improve the Philippines' maritime security capabilities through, among others, transfer of advanced multi-role vessels to the Philippine Coast Guard and TC-90 reconnaissance aircraft to Philippine Navy. Tokyo has long believed that its own security depends on keeping open the vital trade links that path through the South China Sea. Abe's own nationalist instincts have injected newfound urgency into the approach.

Japan is becoming a regular participant in annual Philippine-U.S. military drills. Last year, Japan sent a contingent with an armored vehicle to participate in the Philippine-U.S. Balikatan exercises in the South China Sea.

Bilateral defense ties are set to deepen in the coming years, especially amid the Philippines' multibillion-dollar military modernization program. In an interview on May 28, the Philippine Defense Secretary Delfin Lorenzana told me that Japan was trying to sell Manila its latest radar systems and "looking at the constitution" to see if it could provide heavy equipment such as planes.

If Japan could overcome the restrictions in its pacifist constitution, it should consider negotiating formal defense agreements and even treaties with like-minded nations such as the Philippines. Manila would surely be keen.

Even without constitutional change, the Philippines and Japan should pursue expanded defense and strategic cooperation, including perhaps a Status of Forces Agreement, or SOFA, which would regulate mutual responsibilities.

Support from the Philippines, among the worst victims of Imperial Japan's aggression, would assist the Abe administration's efforts to make Japan a "normal" country in defense terms, and develop security roles commensurate with its economic and technological prowess.

Reinforcing the emerging security cooperation, are the economic ties. For the past half-century, Japan has consistently been among top investors, trade partners, and economic aid sources for the Philippines. As Duterte's trip rightly emphasized, there is no reason for this to change.

For Japan, the Philippines is an obvious destination for diversifying overseas investments away from China amid growing concerns over technology theft, investment protection on the mainland and the trade war.

Japan is the leading foreign partner of Duterte's "build, build, build" infrastructure program. Two of the biggest projects in the Philippines are financed by Japan, namely the multibillion-dollar metro subway project in Manila, the first in the Southeast Asian country, as well as the North-South Commuter Railway (NSCR) project, which aims to connect Manila to its southern and northern-western suburbs.

Despite concerns about inflation in the Philippines and the stability of the government's finances, Japanese investors are attracted to the country's rapid growth, young population and expanding consumer markets in the Philippines. In Tokyo, Duterte collected close to $6 billion in bilateral investment deals. Manila has historically welcomed Japanese investments and infrastructure projects, since they create more jobs for the locals, are of higher quality and based on very low interest rates.

Tokyo also pledged more than $200 million aid for basic infrastructure and peace initiatives in Duterte's home island of Mindanao, where the authorities are trying to suppress Muslim insurgency and create a more inclusive society.

The Philippines president is often known for his mercurial approach. But on Japan he has been consistent. Manila will benefit from his commitment and from Abe's enthusiastic response. So will Tokyo.

Richard Heydarian is a Manila-based academic, columnist and author of "The Rise of Duterte: A Populist Revolt Against Elite Democracy" and forthcoming "The Indo-Paciifc: Trump, China and the New Global Struggle for Mastery."