A trade war is a side effect of protectionism that occurs when one country (Country A) raises tariffs on another country’s (Country B) imports in retaliation for Country B raising tariffs on Country A’s imports. Increased protection causes both nations’ output compositions to move towards their autarky position.
Trade wars could escalated to full conflict between states, as evidenced in the First Opium War which started after the Qing government blockaded its ports and confined British traders, resulted in the dispatch of British Navy to China and engage the Chinese Navy in Battle of Kowloon.
Some economists agree that certain economic protections are more costly than others, because they may be more likely to trigger a trade war. For example, if a country were to raise tariffs, then a second country in retaliation may similarly raise tariffs. An increase in subsidies, however, may be difficult to retaliate against by a foreign country.
Source – Wikipedia.