Victor Davis Hanson, Second World Wars interview with Peter Robinson:

PR: Why is there a plural in the title?

VDH: ...One is that, from 1939, when Germany divided up Poland with the Soviet Union, until... April of '41 - there was a Polish war, there was a Norwegian war, there was a Danish war, a low country war,  there was a French war the blitz, there was a Yugoslav war, there was a Greek war. And all of those together weren't called the second world war. They were seen as isolated border blitzkriegs... And then something weird happened. In 1941 Germany... invaded its de facto ally the Soviet Union... Japan bombed Pearl Harbor... suddenly these border wars where nobody really knew what they were became the Second World War.