Have the Boll weevil crop up halfway through the Civil War. Desperate aristocrats fight harder with less, and thus the south is subjected to a far more radical reconstruction.
If the boll weevil did crop up halfway through the Civil War and it ruined the cotton industry, there is no reason why the South would be subject to a more radical Reconstruction. If the best Northern minds could not solve the boll weevil in 1870 – and I do not think they could – they would likely have encouraged the South to switch to other crops and would not have favoured a harsher Reconstruction than observed.
Alternatively what if the Bill Weevil came up in the 1850s and made slavery useless for cotton prior to a civil war?
Likely, then, slaveowners would have tried to move their slaves into the desert territories of the Southwest, and use them to crop irrigated fruit crops that are as labour-intensive as cotton. Or, if it were possible, they would have sold their slaves as nominally free but extremely low-prices labor in California, where at the time irrigation was just beginning.
My imagination of American history in the event of a boll weevil infestation at the height of slavery is that – whether this be as free or slave labour – there is
an “alternate Great Migration” of African Americans to the Southwest and California rather than to northern cities. Unlike the actual Great Migration, these migrating blacks remain as farm laborers without political rights, and likely would have remained so for a very long time. However, the growth of western agriculture does allow blakcs to form both a dependable labor force and stable communities in the region, and has the effect of reducing immigration from Mexico and potentially from the Philippines, which becomes independent after
World War I rather than World War II. This might mean Japan gains more power in the Pacific, unless (as I imagine would happen) an independent Philippines does turn to the US when attacked.
The problem is that, with blacks unable to move to areas where they can gain even a glimmer ofi political influence as they did in northern cities during the actual Great Migration, what the US would do when and/or if its racism tarnishes its foreign image?? I imagine that under my scenario of a hypothetcal Great Migration to the Southwest, that Northeastern and Midwestern elites are hostile or indifferent to black civil rights there and in the actual slave states. Moreover, with a large proportion of blacks possibly living in less accessible locations, civil rights would be less of an issue to Washington politicians (unless Latin American leaders try to exploit the question). With much greater Federal hostility, civil rights movements would have had much less chance of succeeding.