Often, the best work arises from close engagement with a primary source. As you read, you'll think of questions or begin to shape an argument. The hard part is to find a primary source that addresses the broad general area of interest. Here are some strategies for finding primary sources:
National Archives of the United Kingdom -- many Churchill-era papers are digitized and available for free download
British Foreign Office United States correspondence, 1938-1945: guide to the Scholarly Resources microfilm edition of the Public Record Office collection, text, F.O. 371. Wilmington, Del. : Scholarly Resources, c1979. E183.8.G7 B73 MICROFORM SET AVAILABLE FROM CENTER FOR RESEARCH LIBRARIES VIA INTER-LIBRARY LOAN.
British Foreign Office United States correspondence, 1946-1948 [microform]
Printed guide: Firestone Microforms E183.8.G7B74
"These files contain the dispatches, instructions, reports, memoranda, and other communications that flowed between the Foreign Office and the British embassy and consulates in the United States. The documents give a detailed view of British diplomacy at work and offer much information, gathered by a highly trained diplomatic corps, on internal political, economic, and social affairs in the United States. This microfilm edition, published through exclusive arrangement with the British Public Record Office, faithfully reproduces the organization and content of the Foreign Office Files."
Foreign Office files--United States of America. Series one, USA, politics and diplomacy, 1960-1974 [microform] : (Public Record Office class FO 371, American Department, United States and FCO files from 1967 onwards).
Incomplete contents: pt. 1. The John F. Kennedy years, 1960-1963 : PRO class FO 371/148576-148649, 156435-156516, 162578-162648, 168405-168491 -- pt. 2. The Lyndon B. Johnson years, 1964-1968 : PRO class FO 371/174260-174346, 179557-179622, 184995-185056, and FCO 7/738-884 (reels 27-56).
Firestone Microforms Services (Film) MICROFILM 12020
British documents on foreign affairs--reports and papers from the Foreign Office confidential print. Part II, From the First to the Second World War. Series C, North America, 1919-1939 / general editors, Kenneth Bourne and D. Cameron Watt ; editor D.K. Adams.
Firestone Library (F) Oversize JZ633 .B753 1986q
. Washington: Federal Register Division, National Archives and Records Service, General Services Administration, 1929-2005, 2009
Firestone Library: Non Circulating (Fnc): J80 .U577; also online at http://www.heinonline.org/HOL/Index?index=presidents/pubpapers&collection=presidents. (For FDR see http://www.heinonline.org/HOL/Index?index=presidents/ppafdr&collection=presidents.)
U.S. Department of State
See http://libguides.princeton.edu/history/StateDept for more information
U.S. Military Intelligence Reports, 1911-1944 in ProQuest History Vault
U.S. Diplomatic Post Records, 1914-1945 in ProQuest History Vault