Under WTO rules there is a dispute settlement process which WTO member states are expected to go through to settle trade disputes.
Trump has sidestepped this process by declaring the steel and aluminum tariffs "Safeguard tariffs", which are allowed under WTO laws, claiming that the tariffs are legal under US law based on a 1962 law that allows safeguard tariffs to be enacted for the cause of national security.
I see that many countries have opened disputes with the WTO in response to this. However, several countries have announced retaliatory measures and it is not clear whether these announced counter-tariffs are going to go through the dispute settlement process with the WTO, and what the legal status of these counter-tariffs are in the eyes of the WTO if they are not going through the WTO process.