The US is by far Taiwan's most important friend, and its only ally.
The relationship, forged during World War II and the Cold War, underwent its sternest test in 1979, when President Jimmy Carter ended US diplomatic recognition of Taiwan in order to concentrate on burgeoning ties with China. The US Congress, responding to the move, passed the Taiwan Relations Act, which promises to supply Taiwan with defensive weapons, and stressed that any attack by China would be considered of "grave concern" to the US.
Since then, US policy has been described as one of "strategic ambiguity", seeking to balance China's emergence as a regional power with US admiration for Taiwan's economic success and democratisation.
The US' pivotal role was most clearly shown in 1996, when China conducted provocative missile tests to try and influence Taiwan's first direct presidential election. In response, US President Bill Clinton ordered the biggest display of US military power in Asia since the Vietnam War, sending ships to the Taiwan Strait, and a clear message to Beijing.
The incident may have alerted Washington to the risk it faced of having to fight someone else's war. Two years later President Clinton pledged "three nos" - no to Taiwan independence, no to two Chinas, and no to Taiwan joining international organisations that need statehood for membership.
The administration of George W Bush has appeared more hawkish, even describing China as a "strategic competitor". But it has remained careful not to encourage Taiwan's President Chen Shui-bian, warning him against any actions which risked altering the status quo.