Answer: the decision by the United States government to prevent additional settlement
Explanation:
What was meant by the "closing" of the Western frontier was that the United States government wanted to prevent additional settlement.
This decision was as a result of the government's plan to promote the westward expansion after the Civil War, but however, they had to close the Western frontier to prevent additional settlers.
The immediate cause of World War I that made the aforementioned items come into play (alliances, imperialism, militarism, nationalism) was the assassination of Archduke Franz Ferdinand of Austria Hungary.
George Washington said this quote
