The argument is based on comparative advantage. Using a two country, two product model you should demonstrate that both countries may benefit by engaging in trade when there are differences in opportunity costs of production. If it produces both products then it is allocating resources to an activity which in global terms it is relatively inefficient. If it specialises production then resources will move out of the relatively inefficient activity into the relatively efficient activity. While specialisation will be incomplete there is nevertheless an improvement in the global allocation of resources.
Protectionism is the implementation of policies to erect barriers to free international trade.
They are based upon a number of different arguments and there has always been debate over the validity, or otherwise of these arguments. You should identify some of these arguments, of which the following are the most common. Protection of domestic employment. There is likely to be political pressure for protectionism. The argument that a particular industry may be vital to ensuring future economic and national security.