How should I write an introduction for a term paper on Joseph Stalin's foreign policy?
This depends on what you are going to say in your paper. Your introduction should present the basic idea or argument that you are going to set out in your paper.
If you are writing a paper that simply outlines what Stalin's foreign policy was all about, you should give a brief outline of what you think the main theme is. In other words you should say that Stalin's foreign policy was focused on, for example, expanding communism or on keeping the Soviet Union safe (whichever you think is right). You might allude to what others think and then say what you think.
So you might say something like "Joseph Stalin was one of the most important figures in world history in the 20th century. Many people believe that his foreign policy was aggressive and aimed at spreading communism. However, I will argue that his foreign policy was really about keeping the Soviet Union safe."