We must constantly remind both our Taiwanese friends and those from other countries. The specifics of the answer need to be ever present in the foreground of their sight and thinking:
If China would leave Taiwan alone, the Taiwanese would never choose to invade China or go to war with China.
The problem is, for Taiwan at the moment there is no choice between peace and war. Peace is not the absence of war, but instead true peace is the presence of justice.
China taking over Taiwan would take away peace, democracy and human rights. China taking Taiwan would be an injustice that begets more injustice.
So the choice is between slavery or fighting for freedom -- currently in non-military ways but perhaps one day with weapons of war.
Unfortunately many in Taiwan delude themselves into thinking that being annexed by China is better than fighting for their freedom. People from many other countries share the same delusion about China and Taiwan.
So did Neville Chamberlain regarding Nazi Germany.
Allowing China to annex Taiwan will lead to more war in the future as China's aggressive expansionism and militant nationalism and leninist imperialism is felt by other neighboring countries, and not just ones on the border.
Not only this, but the entire world would be affected as other brutish authoritarian regimes are emboldened by the lack of consequences over China's actions. Many other wars would be started as these regimes think they can get away with anything with impunity.
Allowing China to do such a seemingly trifling little thing (to-some-people) like annexing Taiwan would be the first step down the path to World War III. In this current era of history, if China is not disciplined by the world community, it will continue down the path, and will ultimately be defeated in a world war, but because of said war, there will be untold misery, death and destruction for billions of people. (Note: billions, not millions.)
Nations of the world, do you think Taiwan is a small matter? Think again, very carefully.