The pre-dawn call by U.S. border agents to their Canadian counterparts was shocking: A group of nine people, most of them children, were about to enter Canada on foot.

On Feb. 3 at 6:16 a.m., when the group was spotted, the border between Alberta and Montana was brutally uninviting, covered in snow, dark with a temperature of minus 17 degrees Fahrenheit.

Grainy night-vision images captured by Canadian border cameras showed two little girls in pink winter wear holding a woman’s hand as they trudged through the snow. More children followed in a line. Another adult dragged two suitcases.

The quick intervention by the Royal Canadian Mounted Police crew that found the group was the result of a newly beefed-up border presence across the vast frontier between the United States and Canada. At 5,525 miles, the border is the world’s longest.

Until recently, the border had been described by both nations as “unguarded,” a testament to their close friendship.