5 March Washington Post - Don't Send a Lion to Catch a Mouse by Shankar Vedantam.

...Two political scientists recently examined 250 asymmetrical conflicts, starting with the Peninsular War. Although great powers are vastly more powerful today than in the 19th century, the analysis showed they have become far less likely to win asymmetrical wars. More surprising, the analysis showed that the odds of a powerful nation winning an asymmetrical war decrease as that nation becomes more powerful.

The analysis by Jason Lyall at Princeton University and Lt. Col. Isaiah Wilson III at the U.S. Military Academy at West Point shows that the likelihood of a great power winning an asymmetrical war went from 85 percent during 1800-1850 to 21 percent during 1950-2003.

The same trend was evident when the researchers studied only asymmetrical conflicts involving the United States. The more industrialized a powerful country becomes, the more its military becomes technologically powerful, the less effective it seems to be in an asymmetrical war.

Essentially, what Lyall and Wilson are saying is that if you want to catch a mouse, you need a cat. If you hire a lion to do the job because it is bigger and stronger, the very strength and size of the lion can get in the way of getting the job done...
More at the link.