The Americans faced the most difficult natural obstacle, the dense Argonne Forest.
Instead they went around it, entering France through the Ardennes forest, a thick woodland that the French had mistakenly considered an impenetrable natural obstacle.
But this natural obstacle was nowhere near as dangerous as the area I was about to enter: the "red zone" in Peru is reportedly the biggest cocaine-producing area in the world.
A long squirm down a tight shaft will lead to an even longer crawl, a slippery descent, and so on, in a natural obstacle course, relentless in its challenges.
There is no river between Arizona and Mexico to create a natural obstacle to illegal immigration, drug trafficking and human smuggling, and our county is a major corridor for all these.
Taking stock of this position — which, like a hundred others, he was able to remember perfectly a quarter century later, down to the last log cabin — he concluded that a move at night, to the right, would find "no natural obstacle".
