One of the most important implications of China's rising territorial assertiveness is the transformation of its arch-rival, Japan, into a more self-reliant and confident power. Thanks to its relentless (and often aggressive) pursuit of long-dormant sovereignty claims in the South China and East China seas, Beijing has inadvertently empowered Japan's conservative factions, who have gradually recalibrated the country's age-old pacifism.

After decades of strategic stupor, thanks to its de facto protectorate status under Washington's military shield since the end of World War II, Japan is once again flexing its muscle and shaping the regional order.

But far from assuming the mantle of regional leadership in an aggressive manner, as Tokyo sought to do with disastrous consequences in the early 20th century, Japan is instead becoming a more reliable contributor to the preservation of an essentially American-led order in Asia.

Japan has stepped up its role as a key partner of Asean. South-east Asian countries such as the Philippines and Vietnam, which have been caught in bitter territorial disputes with China and benefited from significant Japanese defence aid in recent years, have enthusiastically welcomed Japan's new regional role.

In the light of lingering doubts over America's wherewithal and commitment to single-handedly rein in Chinese assertiveness, Japan is now seen by many as a much-needed balancer in the region.

Back in 2012, when there was growing controversy over discussions of a re-armed Japan, the Philippines openly endorsed the idea, with Foreign Secretary Albert Del Rosario stating: "We are looking for balancing factors in the region and Japan could be a significant balancing factor."

One person, in particular, has played a pivotal role in carving out a new destiny for Japan. Since his return to power in late 2012, Prime Minister Shinzo Abe, among Japan's most charismatic and energetic leaders in recent memory, has progressively relaxed restrictions on the country's ability to project power across East Asia and beyond.

Under Mr Abe's watch, Japan ended more than a decade of defence spending cuts, commenced exports of advanced military technology to friendly countries, revised guidelines overseeing defence relations with the United States and, just recently, passed a new security Bill which will allow the country to deploy troops for overseas military operations for the first time in seven decades.

As an astute politician, Mr Abe recognised the political impossibility of advocating for an outright amendment of Japan's post-war Constitution, so he instead successfully pushed for a reinterpretation of the Constitution to pave the way for collective security operations.

Under the concept of collective security, Mr Abe envisions the authorisation of military force if "the country's existence, the lives of the people, their freedoms, and the right to seek happiness are feared to be profoundly threatened because of an armed attack on Japan or other countries".

By adopting very broad language, Mr Abe has provided significant latitude for himself and his successors to invoke the right to collective security, a cornerstone of the recently passed security Bill in the Japanese Parliament.

Now, Japan will be able to deploy its Self-Defence Forces if, for instance, an allied nation such as the US is at war with a hostile third party.

With China stepping up its construction activities and military deployments across contested areas in the South China Sea, and openly opposing any American intervention in the area, the probability of clashes between China and the US has gradually increased.

Bereft of the requisite capacity to deter Chinese territorial assertiveness, Asean countries like the Philippines have welcomed Japan's participation in joint patrols in the area. Japan's new security Bill also allows it to support, albeit indirectly, the Philippines in an event of conflict with China.

Under its 1951 Mutual Defence Treaty with the Philippines, the US is obliged to militarily intervene if the troops and vessels of its South-east Asian partner are under attack in the Pacific. Given the depth of territorial disputes between Beijing and Manila, one can conceive of a scenario whereby Japanese forces will be deployed to aid American troops fighting on behalf of the Philippines against China.

Japan's new security Bill sends a clear signal to China that any conflict with the Philippines could trigger a military response not only from the US, but also from Japan.

There is growing hope among South-east Asian countries such as the Philippines and Vietnam that Mr Abe's reforms will pave the way for even more ambitious and comprehensive security agreements with Japan, which could include actively aiding the two Asean countries' domain awareness and coast guard capabilities.

Crucially, Japan has recently signed a defence equipment transfer agreement with the Philippines, which paves the way for Tokyo to supply increasingly sophisticated military hardware and technology. The Philippines is the first country in the region to have signed such an agreement with Japan.

To buttress the Philippines' legal arbitration manoeuvre against China, Japan is also considering taking its East China Sea disputes with Japan to international courts, with South-east Asian countries such as Vietnam and Indonesia having made similar threats in the past year.

To support a more proactive Japanese foreign policy, the Abe administration has pledged US$20 billion (S$27 billion) in aid and loans to South-east Asian partners and up to US$110 billion for high-quality infrastructure development across Asia.

To dispel fears of Japanese remilitarisation, Japan has tried to tone down any bout of historical revisionism within the ruling coalition, with Mr Abe's much-awaited apology statement last August admitting that Imperial Japan "took the wrong course and advanced along the road to war", carefully using terms such as "deep remorse" (tsusetsu na hansei) and "apology" (owabi), and saying that "Japanese, across generations, must squarely face the history of the past".

Japanese Emperor Akihito's recent "peace tour" visit to the Philippines, where he paid respects to the host country's fallen soldiers during World War II, was an unmistakable part of Tokyo's broader efforts to assuage regional anxieties over its recent defence build-up.

As surveys consistently show, Japan is incredibly popular among South-east Asian countries, which seem to have forgiven Tokyo's historical atrocities and appreciated its post-war contribution to regional prosperity.

Now, Japan is also in a position to contribute to regional security and push back against China's territorial assertiveness in the South China Sea.

•The writer is a political science professor at De La Salle University in the Philippines.

• S.E.A. View is a weekly column on South-east Asian affairs.