They were saying the same thing at the turn of the century. The major participants in WW I were all trading with each other. Germany and the UK were each the largest trading partner of the other (or near the top), for example.
War between them was therefore supposed to be impossible.
I remember pulling old articles from the Economist from the first decade of the 20th century trumpeting how economic interdependence was changing the world.
Interdependence makes nations think twice, but in the long run it is not a guarantee that at some point tension and discord cause the system to break down.