Crossing a border is an act of moving from one country to another, usually by land. The term 'cross a border' can be used to describe both legal and illegal crossings. In the context of the crossword puzzle, the answer to the clue 'INVADE' (6 letters) is likely to be 'INFILTRATE', which means to enter secretly or by force. This could refer to a person or group of people entering a country without permission or in violation of immigration laws.