Why was it necessary to form the border patrol?

Because people were crossing. For example mexicans crossing from Mexico without papers  trying to come to the United States.

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The United States learned that the revolutionary army was huge. Furthermore, so many Mexicans were crossing the border seeking farming jobs that Mexican farmers had trouble bringing in their own crops. Americans often crossed the border for the purpose of using counterfeit money and smuggling. The border had become an unsafe place as border towns were infested by bandits and drunkards.

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c) troops march forward in close formation against newer, more accurate weapons