Conversation so far:
How was Thomas Nast's career?
As artist for The Illustrated London News, Thomas Nast's cartoons and articles about the Garibaldi military campaign to unify Italy captured the popular imagination in the U.S.
What else happened that is important regarding Thomas Nast's career besides working as an artist for The Illustrated London News?
Thomas Nast left the New York Illustrated News to work again, briefly, for Frank Leslie's Illustrated News. In 1862, he became a staff illustrator for Harper's Weekly.
Was Thomas Nast popular?
One of Thomas Nast's most celebrated cartoons was Compromise with the South (1864), directed against those in the North who opposed the prosecution of the American Civil War.
Did many people read Compromise wth the South by Thomas Nast?
Thomas Nast's cartoons attracted great attention, and he was referred to by President Abraham Lincoln as our best recruiting sergeant.
Are there any other interesting aspects about the Thomas Nast article besides everything mentioned previously?
Thomas Nast strongly opposed the Reconstruction policy of President Andrew Johnson, whom he depicted in a series of trenchant cartoons that marked Nast's great beginning in the field of caricature.
What happened with the Reconstruction policy that made Thomas Nast oppose it?

Question: What is meant by Nast's great beginning in the field of caricature?
Self-contained rewrite: What is meant by Nast's great beginning in the field of caricature relative to Thomas Nast?