Answer: To keep power and prevent support for the French
Explanation:
When the Naturalization Act was passed in 1798, the United States stood on the brink of war with France. The Federalist party believed that many immigrants would side with France if war had broken out so they tried to limit the number of immigrants that could become citizens so that they do not negatively influence any war effort.
They also wanted to keep power away from the Democratic-Republican party under Jefferson who had massive support amongst immigrants. They feared that if more immigrants became citizens, they would vote the Federalists out of power.
