In fact they can. Somebody who could raise 20x of the entire network's computing power can rerun the transaction history from the beginning, throwing out any transactions they don't like. So long as the rest of the transactions are consistent, they should be able to gain the trunk blockchain line. This would make the entire blockchain we have been running up to this point a very large split.

See What is the longest blockchain fork that has been orphaned to date? for a few details.

In fact they can. Somebody who could raise 20x of the entire network's computing power can rerun the transaction history from the beginning, throwing out any transactions they don't like. So long as the rest of the transactions are consistent, they should be able to gain the trunk blockchain line. This would make the entire blockchain we have been running up to this point a very large split.

See What is the longest blockchain fork that has been orphaned to date? for a few details.