It was big news last month when 7,000 U.S.-trained and -equipped Afghan security troops failed to defend the northern city of Kunduz against a far smaller Taliban force. Yet the setback is just the latest indication of American-trained foreign troops' continuing inability to fight effectively on their own.

It should not have been surprising. Washington experienced this last year in Iraq. The United States spent $25 billion training and equipping a large Iraqi force, which then threw down its weapons and abandoned two key cities, Mosul and Ramadi, to Islamic State militants. Between 800 and 1,000 Islamic State fighters defeated 30,000 Iraqi troops.

This also happened in Vietnam in 1975. There, the U.S.-trained and -equipped South Vietnamese military crumbled in the face of a North Vietnamese attack. The South Vietnamese forces turned that country over to the communists in Hanoi.