There are three problems with the neocon notion of preemption:
1. It places the US in a state of constant conflict because nations will always act to increase there influence.
2. Because preemption involves acting against threats that have not presented themselves, there is the risk of acting against threats that don't exist.
3. Acting to preempt the rise of a nation will be the cause of conflict itself because the nation will have to defend itself against the preemption. In other words, it's a self fulfilling prophecy.