We've never seen a war between nuclear powers. Divisions will just be sitting ducks to get vaporized along with cities and major strategical centers.
Here's the way it'll go down in brief:
China begins to launch an amphibious invasion of Taiwan
US Navy responds after a few of the smaller islands are seized
During some sort of battle of the Fuijan straight the USS Ronald Reagan is sunk
Trump orders that a thermonuclear missile is launched at Shanghai and vaporizes the northern half of the city
After this we're in unfamiliar territory all bets are off. Who knows what happens next but the world is never the same again and it can end with our total destruction. Supposedly astral projectionists from the government can't see anything past middle of May so maybe that's why.