US customs and border protection?
Borders can be secured if the is the political will to spend the money. The DMZ between North and South Korea is an excellent example. With the massive trade between the US and Canada and the US and Mexico, there will always be people and things that get past at busy birder crossings. However, the areas which are not official border crossings can be sealed up to what ever degree the political system is wiling to pay for. Border crossings can be made better with various high tech tools, like back scatter X rays, chemical sensors, facial recognition, good intelligence, etc. So the borders can be secured against serious radiological threats, etc. Keep in mind the the border patrol and customs may not arrest everyone the can at the border, they may let some one through, so they can track them and identify other conspirators and smugglers. So the borders may actually be more secure than they appear to be.