We reconstruct the inherent variability found in mid-nineteenth-century American English by drawing on a corpus of semi-literate correspondence, the Corpus of American Civil War Letters (CACWL), rich in non-standard grammatical features. The primary focus is on a comparison of morpho-syntactic variability (was/were variation and restrictive relativization strategies) in letters written between 1861 and 1865 by Civil War soldiers originating from Massachusetts and Alabama. Key findings include the elevated rate of was-levelling, particularly in the Alabama letters; the variable effect of the type-of-subject constraint on the selection of non-standard was; and the scarcity of WH-relativizers in restrictive relative clauses. Contextualization of these findings in relation to an ongoing quantitative investigation of grammatical variation in four additional states (Pennsylvania, Ohio, North Carolina and South Carolina) represented in the CACWL provides further evidence of structured heterogeneity in Civil War correspondence as well as the sensitivity of variable grammatical processes to regional differences. Taken together, our findings demonstrate how judicious use of the CACWL can leverage new insights into nineteenth-century American English.