Yes, "tend to" is the key phrase, because protectionism tends to reduce the costs of aggression between nations.
Whether importing steel reduces national security depends on many things: where it is being imported from, how much is being imported, what kind of governments do these nations have, and what are the political relations among them? but whatever the answers to these questions, the relevent issue is the resulting trade off. On the one hand, national security may be endangered by importing all steel, but on the other hand, national security has been improved by strengthening trade relations. The question should not start "if there is an outbreak of hostilities ...," but should be "is the reduced chance of hostility worth the extra cost associated with defense in the unlikely case of hostility?"