From CNN,

If China tries to take Taiwan, are the United States and its allies able to stop it? [The] alarming answer is: Quite possibly not. Analysts say China has more troops, more missiles and more ships than Taiwan or its possible supporters, like the US or Japan, could bring to a fight. That means that if China is absolutely determined to take the island it probably can.

But there's a caveat; while China could likely prevail, any victory would come at an extremely bloody price for both Beijing and its adversaries.

Many analysts say an invasion of Taiwan would be more dangerous and complex than the Allied D-Day landings in France in World War II. US government documents put the number of killed, injured and missing from both sides during the almost three-month-long Normandy campaign at almost half a million troops.