It was never ever sanctioned. It just wasn't not sanctioned.

This is just a tiny nit - but slavery was actually sanctioned at the federal level through the Fugitive Slave Law. This was one of the events that led to the Civil War - since the northern states suddenly had to enforce southern slave laws. Northern states who did not oppose slavery (as long as it was below the mason dixon line) and could coexist with the south suddenly opposed slavery since they were being forced to enforce southern laws.

> It was never ever sanctioned.

Yeah, it's not like (for instance) the Constitution expressly forbade Congress (despite it's otherwise general power over international trade) from limiting the slave trade until 1808 except by imposing an import duty capped at $10/head.

Or like the Congress passed, and the federal executive enforced, and the federal courts applied laws protecting slaveholders rights over wayward property even if it strayed into free states.

Except, well...