Trade war - a new style of modern war - between nations

First, why does a country need trading with other countries?
The answer is to exchange for goods and service that it is lacking or short of.

Second, does trading need an international currency as a media?  
No. But, it's convenient to have one, e.g. US dollar, British Sterling, Euro, Japanese Yen, China RenMinBi or China Yuan, or a hard universal currency - GOLD - precious metal.
To avoid some abuse the currency, it may trade for via goods to goods.

Third, is it possible to avoid a trade war?
Yes, but it is hard. The reason is that every nation has its own interests, and it does not want to compromise such interests.

Fourthly, is it possible to balance a trade deal between two nations?
Yes, but it is difficult to reach so as long as there are differences in technologies, labor skills, working ethics, etc. between them.

Fifthly, is there is a winner in the on-going trade war?
No, but one side may benefit more than the other. For instance, the country has a strong currency that can be printed freely, that can freely get goods from the other country if  the latter is still willing to trade with the former. However, such situation cannot continue forever.