Who is to decide what need preventing? There's nobody wise enough to qualify. Not even the President of the United States IMHO.
Somebody already mentioned Vietnam as an example. The murder of Salvador Allende and the resulting Pinochet-rule in Chile could be used as another example of preventive action. The US didn't want another socialist country in their hemisphere so they backed the junta, in deposing an elected leader of another state. This led thousands of people to their early grave.
So my point being, we certainly don't have a good track record with this preventive action.