If an entity that could harness sufficient computing power like the NSA wanted to take control of the block chain by throwing more computing power at it than is

If an entity that could harness sufficient computing power like the NSA wanted to take control of the block chain by throwing more computing power at it than is currently devoted to computing the block chain, then, yes, they would be in control of which transactions were permitted, and could undo very recent transactions that seemed to have been committed. Older transactions would be very difficult to undo, however.