As the tin says, what if Narciso López successfully filibustered Cuba in the early 1850s?
There would probably be a severe political crisis in the US as many northerners would see this as a southern plot to expand slavery.
Also, since President Taylor was actively working against it, I imagine that anyone that got involved (like Davis or Lee in this TL) would face repercussions. The effect this would have on the Civil War would be enormous. I'm surprised I'm not able to find more on this topic...