" if you were to control enough nodes around at least 51% of the computing power couldn't you alter the blockchain to your favor?"


There is already a plausible attack and you just need >= 35% of the hashing power so the current largest mining pool could do it.

The simulator is here:

http://ebfull.github.io/

Basically the evil pool starts to grab an unfair share of the revenue by broadcasting secretly found blocks just before someone else with less connectivity finds a block. Effectively they are exploiting a timing race to tilt the odds and grab revenue due other miners.

This was forshadowed here by a member in 2010 and they also ran some simulations to show the problem, it was re-discovered but with more detail by this guy: http://hackingdistributed.com/2013/11/17/selfish-mining-simulator/