> 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.

Yes. It's a very strange and basically un-contested idea that trade integration discourages war. If that was true then civil wars would be the rarest form of war, but the only type of war going on in the world today is civil war. And if you look through the 20th century, then other than the World Wars, most conflict was civil in nature. You can't get more integrated than a countries internal trade. So this idea that trade deals make war hard doesn't seem empirically supportable.

Good point. The third biggest war of the 20th Century was the Chinese Civil War.