President Trump, using a 1960s Act which gives the President rights to protect national security, to argue for tariffs on steel and aluminium imports into the US seems a stretch but the implications to world trade if carried out are very serious. Much more serious than Russian sanctions or a hard BREXIT. 

Trump threatened in the 2016 campaign to introduce tariffs but it was assumed this was kicked into the long grass when he asked for a commission to report on the issue last year.  However, to people's surprise, the report supported a tariff and President Trump seems determined to follow through on his campaign promise. 

At a time when the OECD and World Bank's projections show sustained global growth, Trump's announcement could grind those projections to a halt.

With his tax reforms there was already concern that the America First policy could distort global M&A activity,  as US companies focus on US domestic M&A but a trade/ tariff war has far more serious consequences.  A tit for tat reaction from China or India or indeed the EU will  no doubt mean a trade war which could expand into other products and services.  

Despite the failure of the Dubai world trade rounds a few years ago, it was assumed a tariff free world could still happen. 

Tariffs and, in particular, using national security to create them is a massive threat to globalisation and will hurt not only the so-called cheap imports it is protecting against, but also the business domestically for which the tariffs are there to protect.  US domestic steel and aluminium production will become a domestic consumption focus and the rest of the world will be open to non-US companies.  History shows the economy of a country without a global trade focus shrinks in the long term. 

The use of national security as a means of economic protection is increasingly being used.  We see it in France as Ministry of Economy approvals are required increasingly in M&A.  We see consents needed or regulations being used to stifle overseas direct investment  in China and Latin America to protect local companies.  Historically the UK has been a liberal economy and the free trade movement still lives strong from the 1800s.  BREXIT mustn't  become a route to protectionism.  The UK can only survive by maintaining its global trade ethos. The US administration should take heed of history.