Historians have always believed that good sources make for good studies. When Lord Acton was planning the Cambridge Modern History a hundred years ago, his view of the massive enterprise was much influenced by the sudden and extraordinary access to historical archives that came about in the 1890s. In his instructions for contributors to the vast effort which he organized but never saw completed, Acton wrote:

Many optimistic historians of and in the twentieth century believed that the events of the 1990s made for a breakthrough in historical knowledge similar to that which Lord Acton and his colleagues had perceived a hundred years before. With the collapse of the Soviet Union, scholars began gaining access to the formerly secret archives of states and governments all over the world – not just because of the fall of authoritarian and secretive Communist regimes, but also because many political leaders in many parts of the world believed that “freedom of information,” as it is now often called, had become an integral part of good governance. While very often producing as selective and partial a documentation as that of the nineteenth century had turned out to be, the new access to information in the 1990s meant real advances for historians, especially those attempting to understand events of the late twentieth century.