The constitutionality of the newly signed pact allowing the US a wider military presence in the Philippines is likely to be challenged in the country's Supreme Court by a group of former senators and other activists who criticised the secrecy of the long-running negotiations.
US President Barack Obama said the 10-year agreement signed yesterday would promote peace and stability in the region and that he hoped China would allow its neighbours to prosper.
But the increased US military role drew consternation from some Filipino activists, who say the agreement reverses democratic gains achieved when American military bases were shut in the early 1990s, ending a nearly century-long US military presence in its former colony.
The criticism also came from opposition members and some within Philippine President Benigno Aquino's own Liberal Party who said they were not briefed on the pact's contents.
On the eve of the US president's visit, former Philippine vice president Teofisto Guingona and former senator Wigberto Tanada signed a joint statement with other religious and civil society leaders criticising the secret negotiations.
Fellow former senator Rene Saguisag said: "For weeks now, we have been pleading on bended knees [to be briefed on the draft pact] ... what this administration is doing is procedurally deplorable."
Left-wing opposition congressmen Neri Colmenares and Carlos Isagani Zarate said they would challenge the pact in court because this was "patently unconstitutional" since Aquino insisted it was an executive agreement and therefore no Senate scrutiny was required.
The Enhanced Defence Co-operation Agreement gives American forces temporary access to selected military camps and allows them to position fighter jets and ships.
In a press conference with Aquino yesterday, Obama made no specific pledge on what the United States would do in the event of a conflict between Manila and Beijing over the disputed Spratly Islands.
In Japan last week, Obama said the United States would come Tokyo's aid in the event of conflict over the disputed Diaoyu islands, which Japan calls the Senkakus. "I don't see how this [pact] can be in our national interest when in the first place the US has never said that they will come to our military assistance if we are expelled from any of the disputed islands in the Spratlys and shoals we hold," said University of the Philippines international law professor Harry Roque, who is joining one of the legal challenges.
"The US has never recognised our titles to the disputed islands and waters on the Spratlys."
However, if Obama were to make the same categorical commitment on Spratlys that he did with Japan over the Senkaku islands, then Roque said he would support the new pact.
Obama said that the US was not trying to reclaim bases. He said the agreement would improve maritime security and hasten response to natural disasters.
Many ordinary Filipinos hold Obama in high regard and a survey released yesterday gave some measure of Filipino attitudes to the United States. Private pollster Social Weather Stations said 85 per cent of 1,200 adults it interviewed had "much trust" in America, compared to 2 per cent who had "very little trust."
Rafael Alunan, a former interior secretary and now one of the leaders of the West Philippine Coalition which often stages protests outside the Chinese consulate, said: "The Enhanced Defence Co-operation Agreement is necessary for our survival and security."
However, about 800 activists burned mock US flags and chanted "no-bama, no bases, no war" on the road leading to the gates of the palace where Obama met Aquino.
Additional reporting by Associated Press
A history of relations between the US and the Philippines:
May 1, 1898: Commodore George Dewey crushes Spain's fleet at the Battle of Manila Bay during the Spanish-American war. In December, the Treaty of Paris ends the war, and the US pays Spain US$20 million to take over colonial rule of the Philippines.
February 4, 1899: A US patrol kills three Filipino nationalists, triggering three years of fighting that left 20,000 Filipino rebels and 4,200 US troops dead.
January 17, 1933: The US passes a law to give the Philippines independence after 10 years of US-supervised self-government.
December 8, 1941: Japanese planes attack Clark Air Base, eight hours after destroying Pearl Harbour in Hawaii, as the US is drawn into the second world war.
April 9, 1942: US and Filipino forces surrender to Japan near Manila. About 12,000 American and 63,000 Filipino prisoners of war are forced into a six-day "death march" to a prison camp in the northern Philippines.
July 5, 1945: The US liberates the Philippines.
July 4, 1946: The US grants the Philippines independence.
August 30, 1951: The Philippines and the US sign a defence treaty.
November 24, 1992: The carrier USS Belleau Wood sails out of Subic Bay, ending US military presence in the Philippines.
February 8, 1995: The Philippines finds structures built by China on Mischief Reef, which Manila claims. The Philippines looks to the US for support.
February 10, 1998: The Philippines and the US sign a Visiting Forces Agreement for increased military co-operation.
September, 2002: US Special Forces troops deploy in the southern Philippines to train soldiers fighting al-Qaeda-linked Muslim militants. This begins a rotating but permanent US presence of about 500 soldiers in the south.
May 19, 2003: George W. Bush designates the Philippines as a major non-Nato ally.