In fact, people thought it was a joke all the times he mentioned it.
So if it does become a real thing, it sets the stage for new “disputes” to be created, which aren’t actually disputes but larger countries deciding to take over other countries’ land simply because they can.
Now that the US is annexing countries again, then other countries will consider as well.
Obviously, China will take Taiwan in this case. But what’s really stopping China from annexing Vietnam? Japan next? SK? They can claim the same “security need” that the US is claiming for Greenland.
If the US does this, the next 100 years will be similar to the 1800s in terms of expansion by super powers.
China will grab Taiwan.
And of course, countries will simply invent new disputes. Trump is already ahead by bringing up Panama and Canada, but more will follow.
These events could very likely lead to WW3 and you want to call that "contained"? Give me a break.