Explainer: Why U.S. seeks closer security cooperation with the Philippines

Vice President Kamala Harris visits the Philippines this week in the Biden administration's latest high-level engagement with America's oldest Asian ally and an increasingly vital strategic partner as tensions rise with China over Taiwan.

Philippine nationalism forced Washington to vacate those in the 1990s, but in recent years the allies have cooperated on counter-terrorism and in response to rising Chinese military pressure in the South China Sea, where the Philippines has rival claims.

plans to deter and respond to any Chinese attack on Taiwan, a self-administered island China claims as it own.

Harris also plans a highly symbolic stop on the Philippine islands of Palawan in the South China Sea to show U.S.

treaty allies in the Indo-Pacific - Australia, South Korea, Japan, the Philippines and Thailand - the Philippines is closest to Taiwan, its northernmost land mass of Luzon just 200 km (120 miles) away.

He said the political environment for greater military access appeared to be improving under Marcos after a rocky period of relations during the six-year term of President Rodrigo Duterte, who sought closer ties with China.

How would a Taiwan conflict affect the Philippines.

Poling believes it would be extremely difficult for the Philippines to remain neutral in a Taiwan conflict given its proximity to the island and its treaty obligations to the United States.

"So if they want American support in the South China Sea, the Americans will expect Philippine support on Taiwan.".

Schriver said that with the Pentagon's rising concern about a possible attack on Taiwan, Washington would want assurances over access within the next year or two, though any overt planning for a Taiwan contingency is highly sensitive for Manila.

"The second Philippine demand is a continued clear commitment to defend Filipinos in the South China Sea," Poling said.

If there was a Chinese attack on a Philippine base in the South China Sea tomorrow, could the Americans actually do anything about it.