In 1849, Venezuelan-born adventurer and filibuster Narciso López was seeking military help for an attempt to liberate Cuba from Spanish rule. Of course, I use the term "liberate" rather loosely, since López was a supporter of slavery. Knowing that pro-slavery sentiment was strong in the South, he decided to try his hand at gaining support down in Dixie. First he solicited aid from then-Senator Jefferson Davis, who declined, but recommended then-Major Robert E. Lee, whom he knew from their service in the Mexican-American War. When López approached Lee, he considered the offer, but eventually also declined.

Given how his OTL attempt turned out, I doubt things would've gone much better for López if he'd managed to recruit one or even both of them. But I'm sure there would be significant butterflies nevertheless. What would be different if he'd gotten one or both of them to join him?