Definition:
The "US Border Patrol" is a group of law enforcement officers in the United States. Their main job is to watch the borders of the country and make sure that people do not enter the country illegally. They work to prevent unauthorized entry, especially along the borders with Mexico and Canada.
"Cross the border": This means to go from one country to another, often used in discussions about immigration.
"Watch over": This means to keep an eye on someone or something to ensure safety.
The US Border Patrol is an important organization that helps maintain the security of the United States by preventing illegal entry.