Several months ago, I praised Matthew Karp's book documenting how pro-slavery southerners, often in powerful positions in the federal government, promoted a military buildup and foreign policies to defend, support, and expand slavery in the western hemisphere.
The New York Review of Books this week has a favorable review of Karp's book, which also serves as a summary of his arguments. [There's a paywall, so use the library, subscribe, or just read the book.]
Among his points: southerners favored and achieved a strong military buildup in the 1850s in part to guard against abolitionist attacks from Britain or its emancipated slaves; while arguing states' rights on domestic policy, they favored a strong central government for foreign policy. Karp also shows a pro-slavery tilt to other American foreign policies in the two decades before the war of the rebellion began.