Firstly he see's the switch during World War I to a more "interventionist" / internationalist stance as being very heavily driven by partisanship. I should emphasise he does not take this to a ridiculous level-he certainly quotes other factors. He also provides some powerful evidence. In the 1920's when Republican presidents suggested their own "internationalist "proposals they got huge opposition from Southern Democrats (which is to say southern congressmen) who tended to actively oppose foreign aid and showed much higher levels of opposition to the World Court than they had to the (more radical) League of Nations. Similarly in World War 2 the fact Roosevelt was a Democrat who had won again and again overwhelmingly victory in the South clearly helped support - republican congressmen in the United States were much less likely to support aid to Britain before entry to the war despite having been if anything the slightly more Teutonophobic party historically. So there is a great deal of truth in this analysis.
I think however he takes it too far. In Several of his examples I think he gives to much weight to partisanship . For example he says partisan motivation (including a love for Wilson as the "southern president") is shown by the reluctance of southern senators to pass the Versailles treaty with amendments to the League of Nations ( the League of Nations and the Versailles Treaty was killed in part by hard-line league supporters the "battalion of death" who combined with opponents of any League to vote down a modified league). However this can equally be seen as a hard-line interventionist position- often measures fail in part because they are seen as too moderate by some people who'd prefer them to the status quo. Similarly he fails to give sufficient weight to the fact that while foreign aid southerners might have voted against in a partisan way they gave much more support to the likes of the World Court (and many of their reasons given for opposition were discontent that it was an alternative to full league membership). And by the late 1930's Southern Senators were happily opposing their party's changes on race- and even siding with Republicans on Some Labour issues- why the likes of Senator Walter George of Georgia who had clashed so bitterly with Roosevelt would back him out of strict partisanship I do not know.
An alternative way of looking at it (which works well for the Wilson administration) is that from the Civil War til the 1930's the south was the Democratic party's core-and with the odd rare exception (1928 presidential election comes to mind) defined their stance-in other words up to a point Wilsonian anti-imperial nationalism and internationalism was the product of the southern political consensus (and much else about Wilson's politics-it was he who killed the anti racism part of the League of Nations Covenant).
And there are many ways it can be tied together. After all Wilson was the one who emphasised self determination famously as a principle in the post war settlement and influenced large parts of the settlement with this principle. He also significantly modified and to an under appreciated degree achieved serious reductions in the harshness of the peace terms for Germany. There is incidentally a reasonable case this was very destructive in the post war era. These themes of self-determination and reconciliation after war (particularly the emphasis on keeping military occupation to a minimum) powerful evoke Southerners of his generation's understanding of key lessons from the Civil War and Reconstruction ( the era after). So Wilson offered a nationalism distinctively different at least in principle from that of a Theodore Roosevelt-an anti imperialist nationalism
At the same time this principle like such fairly vague foreign policy principles can lead to a very partisan spin on particulars. For was foreign aid for Latin America imperialist or internationalist in motive ? This same problem can be seen in the use of partisan cues by Labour mps or Democratic members of congress in determining whether the Iraq or Kosovo wars were "humanitarian" or not - in mattes of judgement the strictest of ideologues can easily end up taking a partisan position. Of course many would argue such distinctions are an absurd one to this day-but that does not rebut the sincerity of such beliefs or the difficulty in operating on the basis of them.
I think this legacy of this ideology can be seen in the speed with which the South backed intervention for Roosevelt (the South supported most aid to Britain by factors of over 8-1 , other regions rarely went over 2-1 in support)-a war which could more easily than World 2 be seen as a simple case of resisting imperialist aggression by Germany. Moreover there was an obvious ethnic factor. White southerners are heavily English in background- by contrast the Midwest the most German-American region was an isolationist stronghold. This helps explain why Southern Democrats were actually more solid than northern in support-and thus the south as a whole massively more supportive of aid for Britain (this of course also feeds to the enthusiasm for the South in World War 1).
Or to put it another way Fry argues against seeing the south as internationalist but with slavery dead and buried there is a strong case that it was naturally inclined to a pro British but anti-imperialist "internationalism"-and this fits well with the record of the South in the early 20th century.
In a latter post I hope to talk about some of the ways this era has implications for Fry's views on the post World War 2 era.