The resulting inefficiency will mean that your population, forced to buy from this firm, faces higher prices than on the world market. Bad news National security. The argument here is that, in case of war, you might need to have a domestic steel industry. So, if we buy all or most of our steel from abroad, we will be in trouble if there is a war and the country we are fighting is the country from which we buy steel. And not surprisingly, that is what happens.
Every industry can argue that it needs protection for national security reasons.
A counter point to this is that, if you trade with a country, you have a mutual interest in each other s well being and may be less likely to engage in conflict with one another. The argument here is that you want to protect domestic workers from dislocation. The problem is that the gains to the entire economy from trade outweigh the losses to this group. So, it is better for the economy as a whole to free trade and just help these workers out by giving them money to retrain or transfer to another profession. This is not as simple as it seems, as older workers have trouble retooling, acquiring new skills.