The northern frontier of the United States is a 4,000-mile stretch of land known as the world's longest undefended border. Just as the terrain is varied, so too are the security standards - high-tech in some places, low-key in others.

Although there has long been a tradition of mutual trust along the US-Canada line, rising concerns over illegal immigration and tensions caused by America's ongoing war on terror are prompting strict new regulations to be put into place.

Over the past several months, Monitor photographers went to five states, to capture reflections of what life is like along the border, and how residents and travelers handle the changing landscape of security. Follow their journey.