Kevin Drum calls into question the idea that assassins usually fail to achieve their objectives, arguing, among other things:
John Wilkes Booth may not have saved the Confederacy, but in the longer term he was probably pretty effective — though I suppose you can always make the argument that things would eventually have turned out the same regardless of whether or not Lincoln had served out his second term. But that's cheating: if you take that view of history, then assassins are ineffective by definition and the game is over before it begins.
For one thing, I think you need to take the Confederates' goals here a little more seriously. It's true that the Jim Crow system that replaced slavery was grossly inadequate from the standpoint of social justice, but it really was a step up from chattel slavery. If the rebels had wanted white supremacy without slavery, they could have gotten that without firing a shot. Indeed, if they'd been willing to accept so much as a prohibition on the further expansion of slavery, they could have gotten that without firing a shot. Confederates wanted the Confederacy, and Booth didn't achieve it.
What's more, though, it's not only "in the long run" that the assassination didn't make a difference, it didn't make much of a difference in the short-run either. Lincoln's death brought Andrew Johnson to power, and he was committed to white supremacy, but by 1869 Ulysses Grant and other Republicans committed to reconstruction were back in office. "Redemption" didn't happen until the 1870s and 1880s and the main Jim Crow laws were put in place in the 1890s. The Supreme Court overturned civil rights legislation in 1883 and Plessy v. Ferguson happened in 1896. The key political battles, in other words, happened after Lincoln would have been out of power anywhere.
That said, Henry Farrell has recently pointed to two papers on the assassination question that I ought to look at. First, Benjamin Jones and Benjamin Olken (2007), “Hit or Miss? The Effect of Assassinations on Institutions and War":
Assassinations are a persistent feature of the political landscape. Using a new data set of assassination attempts on all world leaders from 1875 to 2004, we exploit inherent randomness in the success or failure of assassination attempts to identify assassination’s effects. We find that, on average, successful assassinations of autocrats produce sustained moves toward democracy. We also find that assassinations affect the intensity of small-scale conflicts. The results document a contemporary source of institutional change, inform theories of conflict, and show that small sources of randomness can have a pronounced effect on history.
Next, Zaryab Iqbal and Christopher Zorn (forthcoming), “The Political Consequences of Assassination”:
The assassination of a political leader is among the highest-profile acts of political violence, and conventional wisdom holds that such events often have substantial political, social, and economic effects on states. We investigate the extent to which the assassination of a head of state affects political stability, through an analysis of all assassinations of heads of state between 1952 and 1997. We examine the political consequences of assassination by assessing the levels of political unrest, instability, and civil war in states that experience the assassination of their head of state. Our findings support the existence of an interactive relationship among assassination, leadership succession, and political turmoil: in particular, we find that assassinations’ effects on political instability are greatest in systems in which the process of leadership succession is informal and unregulated.
So with that, happy Veterans Day: heck of a job Princip.