Interestingly, this song was written by Woody Guthrie, who was pretty radical in his time (he was blacklisted during the McCarthy era for his socialist views). A lot was going on in America during Guthrie's life (Great Depression and New Deal, for example).
Here's an article by NPR about Guthrie and the song: http://www.npr.org/2000/07/03/1076186/this-land-is-your-land
It looks like your assignment is to compare time periods. Maybe you could research another time period that provides another anthem? I think a few came out after 9/11. Or maybe another anthem came out during the 1960s (I'm thinking of hippies and their radical movement).
The key with a comparison is to find at least one thing both periods have in common. In this case, perhaps it's easier to focus on radical views (attitudes about what it means to be an American, the values of American society, the effects of government, etc.)
If you use any article as a source, you must remember to document it. Put quotation marks around any borrowed language, and be sure to announce the title of article before the quote. For example:
According to "The Story of Woody Guthrie's 'This Land is Your Land," Guthrie "was irritated by Irving Berlin's 'God Bless America,' ...which seemed to be endlessly playing on the radio in the late 1930s. So irritated, in fact, that he wrote this song as a retort, at first sarcastically calling it 'God Blessed America for Me' before renaming it 'This Land Is Your Land'" (Spitzer).
You will notice that the author's last name is placed in parentheses at the end of quote.
If you need help with citation, I'm more than happy to help!