How big a threat are Trump’s tariff proposals?

It looks increasingly likely that President Trump’s plan to impose tariffs on imports of steel and aluminium on grounds of national security will be implemented in some form. If so, other governments including the European Union, will respond with measures of their own. However, while the EU may move more quickly than it has done in the past, its politicians also have good reasons to exercise restraint in order to minimise the risk of an out-and-out trade conflict. Meanwhile, the bigger concern is of a much broader and deeper shift towards protectionism in the coming years.