China is upset about America wanting a more fair trade relationship which includes fair trade not just "free trade".
Both sides of the political aisle here in the U.S. understand that China's trade schemes are extremely unfair which is why you see Senators like Chuck Schumer of N.Y., a high ranking democrat siding with Trump on this issue alone.
China has been engaged in unfair trade practices ever since it joined the WTO in 2001 and people on both sides of the aisle have recognized it but very few have been willing to actually do anything about it until Trump.
The trade war between America and China began after the Clinton administration and congress back in 2000 granted China "permanent normal trade relationship" status with the World Trade Organization, effectively handing over congress' power over trade policy with China to this globalist trade scheme.
The Trump administration was elected to correct this mistake and now major problem. This is what Trump is doing. Trump is simply attempting to set this trade relationship right and fair.