If I was the most powerful government in the world, I would build a cluster of ASICs which outnumbers the biggest mining pool, and start mining, and start buying coins until I have 2/3 of all coins in my possession. This should not be too expensive for government which collects taxes worth of $3 trillion each year, right? And after I have all these coins, I would convert the rest of the world into slave colony, MWAHAHAHA!!!!
I am a bitcoin supporter, and I respect efforts the developers and community apply to make our world better. Please, somebody convince me that such evil strategy is not executed right now.
P.S. Happy Easter to everybody who celebrate it, and happy weekend to the rest of the world!
Bitcoins get their value from the trust which people put into the network. If a government, or just a single entity for that matter, controlled +51 % of the network, the trust would fall, the price would fall and the owner of a lot of BTCs would lose money. So it is in the interest of all holders to increase the trust in the system. Besides, if Bitcoins fail, a new crypto currency can easily be made.
Edit: But of course some one could have an interest in breaking the system by controlling +51 % and using this to destory the system (not as easy as it sounds and 51 % may not be enough). But this would be very costly.