According to many earlier sources bitcoin must already be dead. They are sure it will not even last a year. In reality the my guess as to the3 biggest threats would be:
1. SHA encryption is broken. Bitcoin would be the least of our problems then.
2. A concerted, sustained international effort by governments to make bitcoin illegal.
3. A collapse of the Internet infrastructure. No network, no bitcoin.
As far as aliens or secret societies... Get back in your Mom's basement weirdo.
1. Even if SHA were broken, nobody can spend my coins. They might be able to hash faster, perhaps take over the network for a short while (it would become obvious immediately). My coins are still safe, and the community would transition to a different, unbroken hashing function. A temporary disruption of this kind, while painful in terms of delayed payments, would not be the end of Bitcoin.
2. The more people (including those involved with governments worldwide) have vested interest in Bitcoin, the less likely a concerted attack is. Most vulnerable days are behind us, and it's only getting better. Gaddafi was a gold bug, no one has outlawed gold; they simply killed him and took his gold.
3. Yes, this would be a problem. However, try talking to people who have been through recent conflicts. Communications infrastructure is surprisingly robust, and disruptions are usually short-lived. Where there is a will, there is a way.