> Right before WW I European economies had become integrated to an unprecedented extent, and people were saying that the integration would discourage war.

And maybe it did; unfortunately, the myth of offense dominance guaranteed that diplomatic crises would escalate uncontrollably into war, masking any effect which discouraged war as a choice in its own right rather than as a preventive measure against the risk that an opponent would choose war.

It's very difficult to isolate and evaluate the effect of one factor.