Since Donald Trump took office last year，protectionism has gained momentum in the United States. In recent months，Trump has announced plans for a series of trade sanctions against China，calling for tariffs on US$50 billion worth of imports followed by tariffs on another US$100 billion of goods. He also has called for restrictions on investments and acquisitions in the United States by Chinese companies. The potential for a trade war has already become a serious impediment to economic globalization and this could significantly affect the global economic recovery. Developments are being watched closely around the world.

The history of trade barriers can be traced back to the 15th and 16th centuries as mercantilism was promoted. Although mercantilist policies have long been abandoned by major powers，protectionist measures have never really disappeared.

The onset of the Great Depression prompted the United States and European countries to erect trade barriers to protect their own economies，and that in turn amplified the economic damage. In the 1970s，there were bitter trade disputes involving the US and Europe over agricultural products as well as iron and steel. Both sides turned to legal challenges such as anti-dumping and countervailing duties under the framework of General Agreement on Tariffs and Trade (GATT). These were in fact disguised duty increases，which appeared to be more acceptable than the simple measure of raising import duties. In the 1990s，the Americans and the Europeans made use of local content requirements and local procurement priorities，further expanding the arsenal for fighting trade wars. At the beginning of this century，against a  global background of excess production capacity in steel，the US and Europe engaged in a trade confrontation over steel. The US conducted investigations into trade practices of the European Union and Japan under the provisions of domestic trade laws，and imposed import quotas and import licensing requirements. The EU reacted with counter-measures of its own ¨C import quotas and increased import tariffs on US-made steel. It also filed trade complaints with the WTO against the US.