Q. What was the impetus for drafting the General Agreement on Tariffs and Trade?
Take a look at the following page from the World Trade Organization's web site: http://www.wto.org/english/thewto_e/whatis_e/tif_e/fact4_e.htm. It includes the following paragraph which seems to address your question: "Meanwhile, 15 countries had begun talks in December 1945 to reduce and bind customs tariffs. With the Second World War only recently ended, they wanted to give an early boost to trade liberalization, and to begin to correct the legacy of protectionist measures which remained in place from the early 1930s."
For more detailed information, I would suggest a book on the history of the WTO and its predecessor, GATT. You can find some of these by searching in the library catalog. From the library's home page, click on the gray "Catalog" tab to the left of the search box in the middle of the page. Choose a keyword search and type "gatt history." On the results screen, see the Location option on the left and select UHD. You should see a few different books that look relevant -- some in print and some are in Ebook format.