The world trading system is on the verge of collapse. President Donald Trump’s announcement last week that the US will raise

tariffs on steel and aluminium imports — ostensibly in defence of US national security interests — threatens much more than the foreign companies exporting to US markets and the American businesses using imported steel. It declares war on the trade system.

Claiming protection of steel is in the US’s national security interest goes against everything that the World Trade Organization stands for. Why stop at steel? In the 1980s, when the US demanded access to Japanese agricultural markets, Tokyo claimed vehemently that food security justified restrictions on the import of rice. Although WTO rules explicitly allow for trade restrictions based on national security, countries almost never invoke it in defence of their trade protection. National security is the third rail of trade law, and Mr Trump just grabbed it with both hands.