The Philippine Supreme Court has cleared the way for US troops to use at least eight military bases in the country to beef up their presence in the region, amid rising tensions in the South China Sea.
Voting 10-4 in favour yesterday, the court said a controversial 10-year defence pact signed in 2014 is constitutional.
The Enhanced Defence Cooperation Agreement is not a new treaty that requires Senate approval but an "executive agreement" that President Benigno Aquino legally signed, it ruled.
Two former senators who had voted two decades ago to evict US military bases argued that the security pact was a "de facto basing agreement" that should be first approved by the Senate.
The legal challenge had stalled the deal for nearly two years.
The Philippines is banking on a restored American presence on its shores as a deterrent to further Chinese incursions into territories to which the two countries have competing claims.
The new defence pact allows the United States to rotate its troops, planes and ships through at least eight bases in the Asean state.
Two of these bases - on Palawan island and in Subic Bay - will give American forces rapid access to the Spratly archipelago in the southern half of the South China Sea, where China is digging in, reclaiming islands from reefs and building airstrips on these islands.
China claims nearly all of the South China Sea, through which US$5 trillion (S$7 trillion) in trade passes yearly. Its claims overlap those of the Philippines, Vietnam, Malaysia, Brunei and Taiwan.
On Monday, Beijing defended its decision to conduct test flights to an airstrip on one of its newly built islands in the Spratly chain.
The US had two permanent bases in the Philippines before a nationalist Senate in 1992 voted to remove them. One was a naval base in Subic Bay, north of the capital Manila, the largest outside the US with an area of 678 sq km, or about the size of Singapore. It is now an economic zone although its harbour is used by US warships making port calls to the country.
The other, Clark Air Base, is now a civilian airport but still serves as a staging area for American surveillance operations.
Under the new agreement, the US will be able to use these two former bases as well as six others on a longer-term basis to allow it to monitor and limit China's movements in the South China Sea.
"The ruling paves the way for the expansion of American boots on Philippine soil, specifically in Subic and Clark, but it is not a game changer," according to Mr Richard Javad Heydarian, a security expert at De La Salle University.
There is nothing in the security pact that "specifically commits the Americans to aid the Philippines in the event of a contingency, particularly if another standoff or conflict emerges in the South China Sea with China", he said.
He added that the deal could even "encourage China to up the ante and further fortify its position on the ground".
Defence analyst Jose Antonio Custodio told The Straits Times: "To read this as the re-entry of American facilities in the scale of the 1980s is reading too much into it."
He said the new defence deal "is a formality to already existing US activities that somewhat went beyond perceived boundaries" already set under other security arrangements between the US and the Philippines.