China and the EU have joined a group of countries asking the World Trade Organisation to investigate the Trump administration’s decision to impose metals tariffs on national security grounds. In a separate filing, the US asked the WTO to review those nations that retaliated against its duties. The move sets the stage for a showdown at the Geneva-based trade arbiter that some fear could either lead to a US exit or a flood of new protectionist measures invoking what has until now been a rarely used national security loophole in global trade rules. Countries so far have refrained from challenging that at the WTO. But in a statement issued Thursday, Norwegian Foreign Affairs Minister Ine Eriksen Soreide said her country and other nations had chosen to request the establishment of a dispute panel at the WTO. We believe that the US’s additional duty on steel and aluminium is in violation of the WTO rules she wrote. So far, Canada, China, the EU, Mexico, Russia and Turkey have imposed retaliatory tariffs on more than $25 billion worth of US goods. The Trump administration previously said there was no basis under WTO rules for the countries to impose their retaliatory tariffs. These tariffs appear to breach each WTO Member’s commitments under the WTO agreement, US Trade Representative Robert Lighthizer said in July. The US will take all necessary actions to protect our interests. The requests will be considered at the next meeting of the WTO dispute settlement body, which is scheduled for October 29. Nine WTO members — Canada, China, the EU, India, Mexico, Norway, Russia, Switzerland and Turkey — have filed initial complaints that allege Trump’s tariffs on steel and aluminium violated WTO rules. But Thursday’s move takes the disputes an important step closer to a formal case.