I'll recycle an old soc.history.what-if post of mine (with the updating of one URL):
***
Obviously, it would have been a lot harder than Hainan (which was much
closer to the mainland and which already had a large Communist guerrilla
force with which the invaders could link up.) But that doesn't mean that
it couldn't be done:
"Rear Admiral Carl F. Espe, the Acting Director of Naval Intelligence,
concluded that the successive evacuations [of Hainan and smaller offshore
islands] had had a 'devastating effect on morale.'1 The U.S. Charge
d'Affairs and the naval and military attaches in Taipei now concluded that
'Taiwan will probably fall to the Communists sometime before the end of
July.'2 Although Espe thought the attaches' estimate too pessimistic, he
observed that 'there seems to be little doubt that Taiwan will in due
course fall into Communist hands.'3
"The Communist drive to seize Taiwan had by then reached fever pitch. U.S.
naval intelligence reported advanced preparations by the Chinese
Communists for an amphibious attack, labeled appropriately 'Operation
Taiwan.' Walter McConaughy, the U.S. Consul General at Shanghai, reported
that the liberation of Taiwan was being trumpeted publicly by Peking as
the nation's paramount immediate mission and one on which they were
staking the reputation and all the resources of the new regime.
"In what would prove to be a prophetic observation, McConaughy reported to
Washington that 'there would seem no avenue left for Communist retreat.
Either they gain Taiwan, or, goaded by bitter humiliation and by Kremlin
propaganda, they must keep it ever before the Chinese people as China's
great irredentist issue and perpetual cause for anti-American vehemence.
Well to remember that Taiwan Irredentism is not Communist monopoly but
popular Chinese national issue.'4
"By the late spring of 1950, the Chinese Communist armed forces were
prepared to carry out the assault on Taiwan, their most ambitious maritime
operation yet. The Communists assembled 5,000 vessels for the invasion by
commandeering freighters, motorized junks, and sampans and refloating
ships that had been sunk in the Yangtze River during the fight for the
mainland. Further, they gathered and trained over 30,000 fishermen and
other sailors to man the flotilla.
"Since the previous year, General Chen Yi's 3rd Field Army, which was
responsible for the assault, had been positioned on the Fukien coast
opposite the large island. The Communists trained their troops extensively
in amphibious warfare and applied the lessons learned from the Hainan and
other island seizures. Despite an outbreak of the Asian blood fluke
disease, which reportedly felled thousands of soldiers, preparations
proceeded apace for the cross-channel attack. Secretary of Defense Louis
Johnson later revealed that between June 10 and June 24, 1950, the
strength of the field army swelled from 40,000 to 156,000 men. Also prior
to June 25, elements of Lin Piao's 4th Field Army moved from south China
to the Shanghai area, where they were positioned to serve as a strategic
reserve for Chen Yi. Historian Allen Whiting, author of the seminal work,
China Crosses the Yalu, relates that by late June, Peking's exhortations
to men in the units stationed opposite the island paralleled in fervor
those broadcast to the troops before the Hainan invasion. In short, the
Communists were now ready to launch the attack on Taiwan and win final
victory in the civil war."
http://www.history.navy.mil/researc...y-history/a-new-equation/invation-patrol.html
See also Denny Roy, *Taiwan: A Political History*, p. 111:
"For the young PLA, a successful amphibious invasion across the strait was
a difficult but not an impossible proposition, especially given the
leadership's willingness to suffer high casualties in pursuit of a
political victory of such magnitude. The Chinese had reportedly mobilized
over 150,000 assault troops and collected in Fujian ports a large number
of motorized junks for transporting these troops by mid-1950. These junks
were difficult to sink with naval gunfire; the shells tended to pass
cleanly through their wood hulls leaving holes that could be plugged.
They were fitted with 40 mm guns that would allow them to put up a stifff
defense against both aircraft and destroyers. The Sino-Soviet alliance
that coalesced in early 1950 increased the likelihood the PLA would
conquer Taiwan. Stalin refused to get directly involved in the invasion,
but agreed to help beef up Chinese naval and air capabilities in
preparation for the assault. United States and British intelligence
analysts predicted the invasion would take place sometime in the summer of
1950; the US government ordered its nonessential personnal on Taiwan to
evacuate on May 26."
http://books.google.com/books?id=DNqasVI-gWMC&pg=PA111&lpg=PA111&sig=rVLcH5SxMpCVwzx-pgDnTVlhvqk#PPA113,M1
https://groups.google.com/d/msg/soc.history.what-if/dvBUxxSTPdg/7s-mBb2_qu8J
***
In the same post, I discuss whether Truman was moving towards an American commitment to Taiwan even without the Korean War. As I note in that post, there *were* some advocates (Nitze, Dulles, Rusk) of a commitment to prevent a forcible Communist takeover of Taiwan, and not *all* their plans were contingent on an anti-Chiang coup. But not only had they not yet won the support of Truman or Acheson, but within the military they faced the opposition of the Joint Chiefs of Staff. My conclusion was:
"In short, the possibility that the US would have intervened militarily
even without the Korean War and even without a coup against Chiang cannot
IMO be dismissed entirely. For one thing, the Pentagon by June was
recommending a military fact-finding mission by General MacArthur to
Taiwan; it might have been difficult to prevent that from leading to a
full-scale military commitment, because once the general set foot on
Taiwan, the prestige of the US would be at stake. For that reason, George
Kennan urged that before a military survey was sent, two conditions had to
be met: the removal of Chiang from power and a commitment by the Joint
Chiefs to 'go the whole hog if required.' (p. 27) (It is possible that by
June Chiang was so desperate that he would have "voluntarily" resigned to
get US backing for Taiwan; supposedly he said as much in a secret letter
to Truman.) But there was certainly no decision for such intervention
before the Korean crisis. The non-interventionist line was weakening but
had not yet been abandoned."