We all know that Uncle Tom's Cabin has been credited with starting the Civil War, and it's author, Harriet Beecher Stowe, is remembered as "the little lady who began the war."
I have read Uncle Tom's Cabin but have not read The Jungle.
From the synopsis of the latter book, I understand that Sinclair's story is about a Lithuanian immigrant who worked in Chicago's infamous Packingtown in the early 1900s. While Sinclair hoped to expose the packing industry's appalling labor conditions, it was his descriptions of the disgusting filth and contamination in American food that outraged the reading public. As a result, President Theodore Roosevelt demanded an investigation which led to the passage of the Pure Food and Drug laws.
A well written novel may not impact a whole nation, but it can have an impact on its readers.