Many countries have lost more secrets to money than to split or shifting loyalties. John Walker, Aldrich Ames, and Robert Hanson were in it for the cash (and in Hanson's case thrills). But there have always been cases of competing loyalties, sometimes sectional (think how much of the Old Army went over to the Confederacy), sometimes international--in the 20th Century commonly either communism or fascism.

The main difference I see now is the ease of copying and publication.